This trick requires you to adjust your game files, so do this on your own risk.

To make copies of your car, first go to the game directory ("...street legal redlinesave00(your place in users menu)").

There should be two files plus other files that represent the cars that you have. Simply copy the file "PlayerCar0" (or whatever car file that you want) and paste them. Then change their names to "PlayerCar1", "PlayerCar2", "PlayerCar3", and so on.

Now go to the garage in the game, and you will find copies of your original car. (To get money easily, you can make copies of your car and then sell them.)