Use OpenIV or SparkIV and replace the file in the vehicles.img file in Your game directory/pc/models/cdimages.

P.S. I just edited this texture to get a more simple dirt texture. I just didn't want the dirt look like the cars have been offroading.
It more now looks like they were cruising around the city in a rainy weather. It's just by my taste, the original dirt textures are still NICE!