Much like you see in any big city Taxi service. Seat back ads for Uber/Lyft Drivers.
A cheap tablet, and an uber driver who sells ads to local businesses. The ads can be location based so they show via GPS coordinates, or even easier, a simple rotation of ads for local merchants.
This would be a trivial programming challenge and coudl be implemented in a weekend.
Driver can sell ads at a rate they find that sells, local merchants can be assured of eyeballs that are almost prequalified.
Driver can provide reports of number of impressions, etc.