Mad Max

Hood ornaments are obtained from destroying the lead vehicle in a convoy. There are 13 convoys around the map, and their locations aren't directly marked, but the routes they take are. The convoys' routes will be automatically marked on your map as soon as you touch any part of any road on their route. As you progress in the story and Wasteland missions, you will unavoidably cross all the convoy routes.

Warning: before destroying any/all of the convoys, refer to the "Up to the Task" achievement solution. There are a number of challenges that are far easier to farm on convoys before you destroy them all.

In terms of the convoy itself, in order to get the hood ornament to drop, you only have to destroy the lead car, which has the convoy symbol above it. To locate the convoy, you'll need to drive on its route until you catch up to it. Once you do, there are typically 5-6 cars in the convoy. You technically don't need to destroy any of the other cars, but doing so can reduce the frustration of trying to take out the lead car and get the ornament it drops. An upgraded Thunderpoon will make short work of any cars in the convoy. Alternatively, you could shoot the driver or tires with your shotgun to disable them, or pull off tires with the harpoon. The lead vehicle always has way more health than normal cars, so be prepared to use at least 4 Thunderpoon shots to destroy it.

Once you do destroy the lead vehicle and the hood ornament drops, you can hop out of your car and pick it up. Note that, originally, the dropped hood ornaments were not marked on your map, but as of the writing of this guide, they now are. So if you die between destroying the lead vehicle and picking up the ornament, it's very easy to locate the dropped ornament to get it.