then this and a tube of caulk? http://learn.adafruit.com/32x16-32x32-rgb-led-matrix/
OP: it'd be kinda pricey for a prototype, and would be relatively easily 'yoinkable' - as in yoinks, mine now. Put the switch on the dash, pigtailed to a car acessory adapter, 20' cable to system sucker-cup'd to back window. Cheap, Protected, Portable. Few enough pins that you could get away with rj45 on each end and couple with cat5.
Has me thinking of a similar item coupled with a ping or something to flip off tailgaters when they get close.