I'm the owner of ProtoCases.com ( a 3d printing company ) and I recommend you first get to know the subject extensively well before diving in. I would look for popular modded cars in local or online groups of fellow car mod enthusiasts. That way you can think about any potentially useful thing you can come up with and go on selling it or many parts online or locally. I wouldn't get a physical location before truly needing to.
One product example would be an ipad/tablet bezel to replace radios on cars. i would then cover them in faux-leather, fabric or paint depending on the car it's going in.
Another one would be custom logos or writing you can stick at the back of the car. Then spray paint them chrome.
It's not complicated to find many 3d printable things that could potentially sell, you just have to know the subject and mostly think about what you would want for yourself or for your friends, etc.
Hope this helps!