I know i've jumped on this tread relatively late on this thread but I downloaded the file nevertheless and it even runs perfectly fine on my laptop, all i did was assign a car paint shader and a blinn to test the MR render and it rendered first time (picture attached).

I see bullet has revamped your scene and render settings so hopefully that has worked for you, but on a separate note may I suggest you splitting up your car panels rather than it being one piece of geometry, you'll find it a lot easer to work that way and your mesh will be a lot cleaner