Plug the red channel of the mask texture to Alpha and then blue to B, grey to A and see how it goes.
About highresshot…Instead of upscaling with highresshot try this; set the new window size to double the size of your desired resolution > play in new editor window > take a screenshot with highresshot 1. Then downscale the shot in photoshop to the resolution you want.
And consider making it a drivable car regardless of the poly count. 