WATCH_DOGS

Honestly, for this, I was sitting at 17/18 for the longest time. To get it to appear, all I did was sit inside of the hideout by the park on The Loop with the profiler running. Sat there for 10 minutes and it showed up. It sometimes is just purely random in this game.

Edit just for fun: Make sure you profile the target after the take down.

how i got from 17/18 to the last one
You may think this is going to tell you a sure fire way to get the last one to show up. Im afraid it's not. It seems various people are finding various ways to have the last one show up. For me however i had completed 17 before i had reached act 4 and then became stuck on 17/18 however i did not freak out, i finished the campaign only to find it still didn't show up. I then decided i wasn't going to worry about it and went about doing other achievements such as the online hacking and went around searching for songs for the songsneak achievement. After an hour or an hour and a half of mucking around on the game it simply just popped up on the screen like a fixer contract or another side mission. Now this isn't going to happen to everyone i know that however i have made a list of all the known ways to get the last one to show up (if you have found another way please let me know).

Getting the Last convoy;

-Time: it seems for some people that it will inevitably unlock after a certain duration (like it did for me)

-Clearing the map of other side missions: look below at CaptHowdy update (however this could be due to time no one actually knows)

-hacking civilian's phones: now this could be the most important one. Im almost certain at least one of the convoy's come from the hacking of a civilian phone. So i personally believe that if you complete 17 without hacking anyone's phone then the last one will require you to get it from a civilian roaming chicago. However if you have hacked someone's phone for it then hopefully one of these other methods will bring it up (this is my personal theory).

UPDATE BY IX CaptHowdy XI
**** for those who are having trouble getting the last criminal convoy to pop I have an answer. I had my 3 friends confirm this. It took a friend a while but it worked none the less. Complete fixer missions one after another. It might be tedious or mundane but all 3 friends confirmed after completing fixer missions one after another the last convoy will show up on your screen, just press down on the d-pad and follow the gps and enjoys the achievement. My theory is that the game will only allow a certain number of side missions on the map at a certain time. By completing fixer missions you are making room for the last convoy mission to show on your map****

[update by DaCoders]
What I did: I went to Brandon Docks, don't know if that is important, and got a message about invading a game and hack someone. I pressed down and found someone. I succesfully hacked him, don't if this is important and when I came back from that Online Game it just popped up.

Keep in mind i uploaded this guide to show the way i prefer to do these missions. YOU DO NOT have to do them this way i find this the best though.

Criminal convoys are side missions that involve stopping a target vehicle and taking down the target. EDIT***** If it says knockdown target this is the non-lethal approach and you must use a non-lethal attack (B button). if it says takedown the target you are able to KILL the target.**** You are shown the path of the convoy on the minimap once the mission is started. Now you will be prompted on screen when a criminal convoy is available however you DO NOT have to do them when they prompt you. if you dont do it straight away it will appear as a light blue circle with a tyre in it on the minimap and the main map (back button). Once on the minimap they can be completed at any stage.

Some missions are obtained through hacking random civilians phones. These are easiest to find if you have the skill that has blue squares on priority hacking targets. If you are having trouble finding the last one or two simply hack as many phones as you can and pray for the best.

Each convoy has a certain amount of vehicles usually 3. 2 of these vehicles (front and back) will contain enemies who are protecting the target/s. The middle vehicle contains the target/s.

Now i have 2 ways which usually work flawlessly and quickly if done correctly

1st method.
-Find an intersection that will intersect the path of the convoy. (make sure its far enough away so you have time to prepare)
-park your car here and wait for the convoy.
-Take cover behind your car and have a gun ready.
-When the convoy is close shoot a tire or multiple of the vehicle the target is in.
-The target will drive for about 50m until the tyre pops and will then get out and try and escape on foot.
-Hop in your vehicle straight away and drive close to him get out and chase him down holding RT+B. you can then either kill all his back up or drive away.

2nd method
-Find a car or truck (NO motorbikes)
- Try and get to a high speed and ram into the target vehicle, if this is done correctly the target vehicle will stop immediately and the target will get out.
- drive 20 odd metres away and take cover behind your car (you will be getting shot at)
-shoot all the enemies (EXCEPT for the target)
- Chase down the target

Using lethal takedowns (killing the target when allowed)
-Simply place IED in a place where the convoy will pass
-hide somewhere nearby and wait for convoy
_wait till the target vehicle is on top of the IED and blow it up.
-either kill all enemies or drive away


Cheers to Lavindathar about the tip with the lethal and non-lethal takedowns.
Happy Gaming

I was also sitting at 17/18 for what seemed like a long time. Not sure if it was a coincidence, but i began working through the fixer contracts at Brandon Docks and after completing two, the convoy appeared.

Video of the fixer contract i'm talking about, might be helpful. Credit goes to: Taner OZCAN