Did some test with another model, removed de default material from all the faces and it worked.
I found that if the back face is also a default material (the blue face) with will also mess the proxy. I used the UV Toolkit from thomthom to aply de front material to the back face and got a much better result on this other model, only the rims changed material.
Result
http://imgur.com/a/peTIQ
Plugin
https://sketchucation.com/forums/viewtopic.php?t=18992#p155759
For the BMW model I used the Global Material Change from TIG to replace the Default material with something else, and worked fine.
Result
http://imgur.com/a/9CdJ1
Plugin
https://sketchucation.com/forums/viewtopic.php?f=180%26amp;t=13192