It can be done with a mod.

If you want to change paint jobs and parts,you would create a custom AI set that included all of the cars that you want in it.

AI sets go in
data\property\career\ai_sets
they end in .aist
It's basically a list of which cars to use and which version of each car.

The AI sets point to files in each cars \ai\profile\ directory. The files end in .aicr
Make a copy of one that is already there and edit it.
These files determine which skins are used and also which file to look in for the parts list.

If you want to change which parts are used on a car, that file is in
data\vehicle\whichevercar\career\
Either edit default.cavs or make a new .cavs file.

If you get stuck, just ask.

By "no cosmetics", do you also mean the paint jobs or do you just mean the parts?

If you only want to change the parts, the just edit the .cavs files in each car's \career\ directory.

Originally posted by TestTestTest:

By "no cosmetics", do you also mean the paint jobs or do you just mean the parts?

If you only want to change the parts, the just edit the .cavs files in each car's \career\ directory.

I mean I don't want the AI to have any cosmetic or armor, just stock cars, different paints are completely fine.