If somebody can write for you a module that have this functionality where you choose the sticker placement and design and probably make few modifications to the design - it will be the best. I have not seen ANY decent quality custom design module for PS and TB (I mean like to make own custom Tshirts, prints, etc)...
The other workarround I proposed will be tons of work. If I understand your idea - you need your designs to be with the shape of some car panel, let's say fender, then on top of this shape the customer can add custom design so you can print the foil and send it out? In the case I propose you have to split your catalog in brands, then models, then car parts and for each of them you can add different attributes with dropdown menu which can change the design in the product page (in each combo you can have different color, different design, etc).
But imagine you have 250 car models with 5-6 parts per model and then 10-15 designs... A work for whole year to put this in your site.... And after that you will have to manually do the designs for printing/cutting...