[Benjamin] was working for a GPS company called Nemerix about two years ago and decided to hack together this iPod accessory GPS unit. He is using an Atmel ATmega324 and a Nemerix GPS unit, though he says the GPS unit shouldn’t matter. He is taking advantage of the AiR mode on the iPod which allows a 4 color image to be sent to the screen. There was little information on how to utilize this, but BMW and Alpine had made some car audio that featured this. [Benjamin] picked up an Alpine with this function and sniffed the data stream to get what he needed. He says there is good documentation out there now though.
He says that the source code is available if you ask.