So what other hardware is required other than the empeg? What is its cost and its form factor? Would one need to hand-construct any sort of complex interface between the GPS module and the player, or is it a straight serial connection?

Here is what I am using and their approx prices...

Garmin eTrex GPS ($100)
eTrex serial/power cable ($25)
Null Modem adapter and gender changer to plug it into empeg ($7)

Nothing else is required. Nothing needs to be fabricated. Now it would be nice to patch the power cable into the cars electrical system (rather than using the cig lighter) and to rig the GPS to automatically come one with the empeg. These would both require a bit of custom wiring. The GPS I am using is a little bigger than a pack of cigarretes. You could get a cheaper GPS unit meant specifically for this type of application without a display, etc. I think someone posted a link to one someplace on this site a while back.

-Mike