I thought the empeg harness had a serial connection that came off of it.

Yes, it does.

I thought that this serial connection provided power as well and that this was how GPS's were hooked up to the empeg for all the projects.

One of the wires on the serial port is the same wire as the empeg's Amp Remote wire. This is not a standard part of a serial connection, it's something the empeg guys added themselves.

Your Garmin GPS unit probably does not have a power input on that pin of the serial port. So if you decide to use it for 12 volt power, you must make sure that:

a. Your Garmin is designed to run off of 12 volts, and

b. you hack the Garmin, or create some kind of a patch cable at the Garmin end of the cable to connect that wire to the Garmin's power input.

When I spoke earlier about using the Amp Remote wire, it applies the same, whether you do it as one of the serial pins or you do it as a loose wire. Either way, same wire, connected to the same electronics on the empeg.