Like Shift and Shift 2, pCARS reads not only binary materials, but also xml materials which is easier to edit with Notepad. So if you're a modder, copy some of these materials to your car folder, rename them and edit mainly the texture names and then the meterials are done. The game reads mtx files directly without bmt files.
Note that if you use Zmodeler to export models, it automaticly generates mtx materials (which are usually useless). Be careful not to let those generated ones cover your working mtx files!