Car Drops are random points that appear in the world in different locations from game to game and each drop can be only used once per game so make sure you get to them quickly before someone else steals them from you. A Car Drop is identified by three blue flares, three drones and three banners. Driving up to a Car Drop and pressing
will change your car into the one shown. Cqr Drops may either give you a better car or a worse car, so unless you just want the achievement, don't bother going for the worse ones. The achievement will pop up eventually after you play more of The Eliminator and go for other achievements and rewards. Depending on your luck you may get 0-2 per game, maybe more, using 100 of these will unlock the achievement.
According to HomelyBumRocket, some notable areas where you might expect to find car drops are:
• Mortimer Gardens
• Bamburgh Castle