Sicky, I looked at your scene and saw that all of you car was in the Wheels layer. I took everything out of it and added the different parts to layers you already had. I tried to follow the names of your layers, and they all seem to work fine.

I tried posting you file back here, but tells me it is too large. Is there a email I can send it to you?

Also noticed where you copied your car across (like the trunk) need to have some vertices merged.