You have to write that script yourself.
Open the manual and read about the following instructions:
var chosen_car;
If you want 3d car selection: event_click / enable_click.
2d car selection: panel.
Then mouse_mode or the like.
The car or panel event sets chosen_car accordingly.
In game you use if/else and ent_morph and check chosen_car.

If you have problems with your code then I will help you with pleasure.

There may be a car or player selection in a AUM magazine, too.