I've had to use testingcheats to remove stuck police cars, fire trucks, and ice cream trucks on many occasions. I've not had it corrupt any of my games, but I believe that if you did it frequently, it's possible that it would. I suspect I've not had a problem because I only do it on the rare occasion that resetting doesn't work. But to be safe, I always back up my game before doing it. To do it, just turn on testingcheats, shift-click the car, and Object -> Delete.