Easy installation Method
You can easily drop one Mailcar_Diff.dds/passengerCar_Diff.dds into you Custom Assets folder of SMR!
That will overwrite the texture ingame and you will see the "new one" instead of it.
Advanced installation Method
If you are planning a mod or map without overwriting the standard textures in SMR!
i created a new set of kfm and nif files and a snippet of the needed xml code.
If you are using that textures for your map or mod do not forget me and my work
Kindest Regards
PaF Daddy