Car Washes / Dirty Vehicles
Hire a Mixer

Pretty simple idea. Cars should get dirty based on how long they've been travelling, usually moreso in dirty sectors. Badlands should get your car filthy pretty quickly, a full trip around Red might get it dusty, a few days of driving on Gold might get it dusty, or a week on Green. You can clean it yourself, but if you're higher class, why not hire a mixer to do it for you?

Could not find an existing suggestion for this.

I literally put in dirty vehicles into the search and it turned up nothing, that's great.