It is the same as any other car wiring. The Cigarette lighter supplies 12 volts.

If your Garmin takes 12 volts DC (should be printed on the unit somewhere), then you can wire it directly to the same 12volt wire that your empeg is connected to.

If it doesn't draw too much current, then you might also be able to connect it to the empeg's blue amp-remote wire.

Do you intend to permanently mount it in the car, or do you intend to take it out of the car?