You're obviously missing at least one .HDC file (boy, I'm observant this morning...), which, if I understand the file structure correctly, is the backbone of each car (side note - if the car isn't in the showroom view, it's not installed correctly). I'd go back and look at the download files...if the .HDC isn't in there somewhere, the problem is found. What to do about it from there, I dunno...aside from trying to contact the mod builder, but if there wasn't an answer to this in '09, you may be out of luck.
- edit - do you also have the Protoracer Revolutions Mod installed? Apparently the GrandAm mod is also dependent in it.