Verifone has built a network that allows for dynamic advertising within taxis. While not exactly what you're talking about, you should check it out (and similar offerings for the taxi market) as there is a lot of overlap: http://www.verifonemedia.com/
Verifone has a lot of credit card terminals in taxis... why do you think so many have large color screens?
I like John's suggestion to target the Ride Share market as there are fewer entrenched players competing for in-car advertising space.