Dirt effect of the car, must to be a part of environment mod (not a car mod), for at least partially, because Dirt It IS a part of environment. I suggest to use two masks instead one, also this second mask can control the "Stickiness" of substance on which car rides (Grass, Sand, Dirt, Dust, Snow....)
Here is my idea of advanced car-dirt shade/masking:
Also will be nice to move other car effects textures to the environment folder, because this effects are belong to environment much more. Texture list: DirtGravels.dds, DirtMarks.dds, DirtSmoke.dds, GrassMarkFenceIntens.dds, GrassMarkGround.dds
