Saints Row: The Third Cheats
At the moment I haven't come across any actual PC mods for SR3, only forums talking about making them. :/ so eventually mods for it on the pc will pop up.
So all I can say is be patient and keep looking around.
Note: This works for all NON customizable vehicles."
No clue if it'll work on the PC version, but if you have a PC 360 controller you can give it a go. Or press the corresponding buttons on the keyboard/interface (Generally A is accept, B is back.).
EDIT: This is a little easier to see how:
EDIT2: Okay, confirmed this works. Make sure you have your chosen non-customizable car stored in your garage, then take an ordinary, customizable car to rim jobs and hit E. Then select Garage, then whatever car you want, including the non-customizable ones, and select Customize.
[Edited by Skyheart, 12/27/2011 4:42:48 AM]