I do not think Car Manafacturers would make a fuss of you designing thier car in Maya or any 3D package. To ask would just be a matter of courtesy.

They would make for of a fuss if it was a non released design of a Concept Car that you got hold of. But for current models, I cant see any issues. Send a quick email to the press contacts and ask if they can give you an OK. Really people model this type of thing all the time, and most never ask.

As far as I was aware he would have to tip his hat at least to the record label to use the instrumental, unless he changes it in someway.

Weird Al Rocks...