Crackdown

Alright, there are a number of vehicles that need to be impounded in order to get this achievement. I will list them off and try giving some hints as to where to get them. If you're able to, co-op with someone who already has the achievement. By using this other person, they can pull the car out from their garage and drive around Pacific City. You CAN'T just take the car they pulled out and put it in your garage. However, the game spawns cars more frequently if a player is driving one. So, say I'm driving a Pickup. Chances are, I will see a bunch of Pickups.
If you have someone to co-op with but they don't have the achievement: work together to get cars. Try going for different ones. If one of you impounds a car neither of you have, both players will recieve that car in their garage. It also might help to turn on the Gangs in order to get some cars, but it might not be necessary for most of the vehicles. Another note, the condition of the vehicle doesn't matter when impounding. If the car blows up, carry the frame back to the garage and throw it in there.

Now, for the cars themselves...

PEACEKEEPER
-Dispatch Vehicle
You will find about 10 of these bad boys surrounding the garage door.

LOS MUERTOS
-Modified Hitchback
Sometimes spawns outside Tunnel 1. Can also be found near most boss locations (Guerra, Martinez, Diaz, etc)

-Modified Sportscar
See "Modified Hitchback"

-Pony Car
I found this near where Martinez is located. You might confuse this one for the Muscle Car since they look similar. This one doesn't have the engine popping out of the hood.

-Muscle Car
Another common vehicle. See "Modified Hitchback"

-Modified Saloon
A little less common, but can still be found near most boss locations.

-Modified Van
I have seen this spawn near Diaz a few times, so you may be able to find it there (like at the entrance of the arena).

-Slammed Pickup
I have only seen these whenever I had ressurected the gangs. They are the rarest of the Los Muertos vehicles, so if you see one, go and get it. They tend to come up more when there are hit squads.

-Lowrider
I have seen these spawn near Violetta's location. They seem a bit more common if gang hit squads are coming at you.

THE VOLK
-Luxury Saloon
A pretty standard gang vehicle; common to find.

-Popular Saloon
I have seen this spawn in Natalya's location. These look very small compared to the Luxury Saloons.

-Limousine
I'm sure I saw these without the gangs being around, but I primarily spotted them when I didn't have gang members trying to kill me at all.

-APC
It's kind of hard to miss this one. It's only the large vehicle with six wheels. Check Igor's location if you're having trouble finding it, as well as Olga's.

-Military SUV
This one is also kind of common. It's the smallest of the Volk vehicles and it looks like a Jeep.

SHAI-GEN
-Sportscar
I found these parked in front of Mrs. Timbol's location, across the street from the Knowledge Globe.

-Sedan
These are pretty standard vehicles. They also spawn underneath Cowell's tower.

-6WD SUV
These are hard to miss since they got six wheels. They spawn in Thadeous' building, underneath Cowell's tower, and in front of Mrs. Timbol's building.

-SUV
This looks like a regular pick up. I have seen this spawn underneath Cowell's building, next to a semi-truck.

-Limousine
One of these is bound to spawn in front of Thadeuos' building (before the tunnel that leads to the inside).

CIVILIAN
-Cabriolet
These look like Dodge Neon convertables. They are common.

-Sportscar

-Coupe

-Sports Pickup
This pickup should be noted by its 4 doors. Pretty common.

-Microcar
These are the silly looking mini-cars They seem to spawn more in Shai-Gen then anywhere else, but they are still pretty common.

-Luxury Sportscar
I've seen these more in the northern-most island of Shai-Gen than anywhere else. Check near the freeway.

-Baggage Carrier
-The only place I have seen this is on the oil refinary where The kingpin of The Volk is located. It spawns whenever it feels like, so it might not always be there. You have to carry the vehicle from the oil refinary to land. The only tricky part is walking on the tube that's touching the water and trying to get the vehicle on solid ground. I suggest that after the tube, you try heading towards the right and up, and throw the vehicle onto the rocks. Then throw/kick it on flat ground.

-Pickup
A two door pickup. I've seen these more on the freeway. Uncommon.

-SUV
Another uncommon vehicle. Looks very similar to the Miliarty SUV.

The popular saloon, baggage carrier, and modified saloon, are the three hardest to find. All the rest you can pretty much find laying around or gang members are driving them.

To get the modified saloon, you will have to turn gang resurrection on, if you have already beat the Los Muertos, and gang members will be driving it around.

I found the baggage carrier in the Shai-Gen Vitaliy Rzeznik Stadium, first floor next to a dumpster.

I found the Popular saloon pretty easily. Start out a the Volk supply point next to the oil rig. Go to the bottom right corner of the Volk island where the large statue of the hand and ring is. From the hand and ring, go under the overpass for the highway and jump the fence, there should be a popular saloon parked in the parking lot.

HybridPK's solution was a good start but MEA7HOOK's guide on x360a goes a little more in depth. Helped me find a few volk cars that I couldnt for the life of me find.

**All credit goes to MEA7HOOK**

Gang Vehicle Locations for Confiscator
Here are the locations of the gang vehicles that you need to impound for the confiscator achievement. This list does not include civilian vehicles (which you will also need for the achievement) because they aren't necessarily found in specific locations, so make sure you're on the lookout for these as you're collecting gang vehicles.

*The slammed pickup and lowrider from Los Muertos have been giving people trouble and are hard to find after you kill all the bosses and eliminate all the gang members. For every other vehicle you will have the most luck following this guide AFTER eliminating all bosses.

LOS MUERTOS:
1. Modified Hatchback - Go to the dome/Diaz boss location. In the small parking area just outside to the right of the entrance. Can also be found inside the dome towards
the back right.
2. Mod. Sportscar - Same as #1.

3. Mod. Van - Same as #1, but is only found inside the dome just as you enter - next to a big rig.

4. Mod. Saloon - Behind Guerra's club (boss fight with Guerra) there are 3 levels/tiers of parking. This is on the bottom level.

5. Pony Car - Same as #4, but its kind of hidden to the right of two yellow dumpsters.

6. Muscle Car - Parked outside the entrance of the Garcia boss location to the left of the fountain.

7. Lowrider - These were all over the place when the hit squads came after me. If you just want to grab one quick, continue to follow the road after you exit the agency. The road will curve around to the right and you come to a 4-way intersection. Go through the intersection and look immediately to your left in the fenced in area. One will be parked in there.

8. Slammed Pickup - This one gave me some trouble. I got on top of the clown at the entrance of funland and got the gang hit squad after me. Shortly after, I looked down and to the left and a slammed pickup was parked against a wall. I was originally playing with one of the extra players that was available through DLC. After resetting the game 3 times to try and find the pickup without success, I reloaded a 4th time and chose the original black guy agent. Don't know if that matters or not, but it's worth a try because that's when I finally found it.
(Alternate advice for slammed pickup from mchannon37: Spawn at the northernmost supply point then stand in the center of the freeway.)
*For #7 & #8 - Resurrect gangs if you have already killed Los Muertos bosses in La Mugre

VOLK:
1. Luxury Saloon - Start in agency garage and go through tunnel 2 to Volk. Turn right and enter the puregy factory/Sovetnik boss location. 4-5 of these are parked along the road throughout puregy.

2. Popular Saloon - Spawn at "Storage Facility" supply point. Face the oil rig, then head left towards the building that is connected to the rig by the black pipe. On the other side of the building is an orange bunch of tubes that wrap around all the way to the road and continue down the street. Another way to identify the building is that is has 5 red rings around it. 2 popular saloons are parked in front of that building facing the street.

3. Limo - Get on top of the "city coaches" building - same place as the Biragov boss location - you can tell it by the yellow roof. Look south towards the oil rig and walk to the end of the roof. Look down and you will see a fenced-in area with a green building and a row of covered parking spaces. Should be 2 limos in parking spaces.

4. APC - Same as #3, but instead of going to where the limos are, just drop down off the top of the "city coaches" building. (can also be found at same location as #5).

5. Military SUV - These are plentiful in the ship/container yard near "The Docklands" entrace closest to the "Storage Facility" supply point. To get to that entrance, head South/Southwest from the storage facility supply point.

SHAI-GEN:
1. SUV - (This vehicle actually looks like a truck) Go to the front entrance of the Central Point tower. This is the building with the Cowell boss fight and the elevator - it's opposite from Wang's tower. On the ground level, below the entrance that is patrolled by peace keepers, you will see a semi-truck and the SUV blocking the road to a small parking area underneath the tower. Grab the SUV.

2. 6WD SUV - Same as #1, but go past the SUV and semi-truck location and a few of these should be parked just ahead on the left side.

3. Sedan - Same as #1, but go past the SUV and semi-truck location and a few will be parked just ahead on the right side.

*For 1-3 - try killing Cowell to get the vehicles to spawn if you haven't already.

4. Limo - Spawn at "The Block" supply point. Head Northeast towards the PCN Expo Center/Oakley boss location. When you are about halfway to the expo center, look for the underground entrance that reads "PCN eXpo Center". The limo is parked just outside the entrance. You should also see the sportscar.

5. Sportscar - See #4 - parked side by side with the limo.

Please add on if you find a vehicle in another location than what I have listed.

How to impound/where to take the vehicles:
You bring the vehicles back to the agency garage, which is the starting location where you can choose to drive the Agency SUV, Supercar, or Truck. Just drive back through the tunnel that leads to the agency (the "island" in the middle of the other three areas - volk, los muertos, shai-gen) and the door should open for you to drive the car right in. After that, you can walk back up to the garage door and a menu should be available for you to choose which car you want. From that menu you can scroll over and the vehicle you just returned will be available for you to drive, while vehicles you still need to impound will be grayed-out until you return with one.

*Note: If you blow up a vehicle by accident while in route to bringing it back for impound, it has been confirmed that you can simply pick the vehicle up and carry it back to the garage - It will count the same as if you had driven it back to be impounded.

I found the baggage carrier in the Shai-Gen Vitaliy Rzeznik Stadium, first floor next to a dumpster.
to add onto this the boss must be there or else it will not spawn if u try going there when the boss is dead. its whats happened to me all the time until i found this out...

Baggage Carrier - This is the slowest vehicle in the game, and one of the rarest, only appearing in two places depending on your progress. There are two of them sitting on a platform with a large number of fuel barrels on Golyak's oil rig, but they only appear there AFTER he is dead. This is advantageous because detonating the barrels destroys the vehicles (remember, though, that carrying a destroyed vehicle back to the Agency garage is still a viable way to impound it). To retrieve one of these vehicles from the platform, jump down with it in hand until you reach the "front gate" at sea level. Carefully walk along the black pipe holding the vehicle until you are as close as possible to the shore. Make a running leap for the shore on the left side, aim up, and mash the right trigger until the Agent throws the vehicle. It should land up near the street, ready to be driven away. The only other place to find it is in Rzeznik's parking garage in the Corridor, which is accessible via the streets, but it only appears there BEFORE he dies

Baggage Carrier location