Follow these instructions to get a bullet proof car:
1) You must have the minigun. Here are the places it can be found on the map: http://en.wikigta.org/wiki/Minigun_locations_%28GTA_San_Andreas%29
2) Enter this cheat: L1, L2, L2, UP, DOWN, DOWN, UP, R1, R2, R2.
3) Get a car.
4) Shoot the car with the minigun. It should explode once. Before it explodes again, get inside the car.
5) Deactivate the cheat. (You should now be driving a car which is on fire and looks like it will explode.)
6) Enter the following cheat to repair the car: R1, R2, L1, X, LEFT, DOWN, RIGHT, UP, LEFT, DOWN, RIGHT, UP.
7) Drive the car into the garage at one of your houses.
Now, try shooting the car. It should be completely bullet proof.
Note: This doesn't make the car crash proof. If you crash the car enough, it will explode. Also, your cars tyres can still be burst with gunfire.