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...
You will find about 10 of these bad boys surrounding the garage door.
Sometimes spawns outside Tunnel 1. Can also be found near most boss locations (Guerra, Martinez, Diaz, etc)
See "Modified Hitchback"
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.
Another common vehicle. See "Modified Hitchback"
A little less common, but can still be found near most boss locations.
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).
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.
I have seen these spawn near Violetta's location. They seem a bit more common if gang hit squads are coming at you.
A pretty standard gang vehicle; common to find.
I have seen this spawn in Natalya's location. These look very small compared to the Luxury Saloons.
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.
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.
This one is also kind of common. It's the smallest of the Volk vehicles and it looks like a Jeep.
I found these parked in front of Mrs. Timbol's location, across the street from the Knowledge Globe.
These are pretty standard vehicles. They also spawn underneath Cowell's tower.
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.
This looks like a regular pick up. I have seen this spawn underneath Cowell's building, next to a semi-truck.
One of these is bound to spawn in front of Thadeuos' building (before the tunnel that leads to the inside).
These look like Dodge Neon convertables. They are common.
This pickup should be noted by its 4 doors. Pretty common.
These are the silly looking mini-cars They seem to spawn more in Shai-Gen then anywhere else, but they are still pretty common.
I've seen these more in the northern-most island of Shai-Gen than anywhere else. Check near the freeway.
-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.
A two door pickup. I've seen these more on the freeway. Uncommon.
Another uncommon vehicle. Looks very similar to the Miliarty SUV.