You're right, but I don't know of too many 8 channel GPS receivers for sale with serial port and 12 volt interface for $15 each.

If you know of some, then post here.

Still, the thing with GPS, is that the serial interface and power requirements to interface your GPS to the Empeg keeps the "tyre kickers" out - those that can do this sort thing will, those that can't can find someone who can do it for them, and those who only want a "finished" solution can go to Garmin or whatever and plunk more than they probably paid for their [firesale] Empeg in the first place!