My cat loves sleeping on my car, for the heat, or the view, i don't know. Doest matter if its in the garage or outside it. I don't sweat the paint, over the years we have had cats sleeping all over the cars. There may be a scratch from them sliding off but a quick wax always fills them in…But the one thing i do worry about is them punching through the soft top. So I put a towel over the soft top and then a car cover over that. Now for my obligatory car pics.
To solve your problem maybe put one of these mats on your roof, my friend did this to protect his new leather couches and then he didn't need to even turn them on after the first few times.
Amazon.com : PetSafe 60-By-12-Inch ScatMat Electronic Pet Training Mat, Sofa, SKM-C471 : Pet Deterrent Mats : Pet Supplies
Here's him on top of the boxster roof digging in, but he can't get through the towel/cover layers
