Hi bajaru,
I recently fitted a Garmin to my Gen 1, and decided to use one of the spare terminals on the fuse box as a switched power source, only on when the ignitions on, and once done you have a go to power source should you want to fit another device at a later date. Mines a 2005, but pretty sure the wiring diagram is the same for 2003 onwards. If you remove the fuse box from its mounting under the seat and look underneath, you'll see six sections, two of which don't have wires in. I used the third from the left as viewed underneath. If you remove the fuse it's possible to prise out the fuse holder which will have two terminals, one spare and one with a wire on. Solder your GPS power lead, which will have its own fuse, to the spare tag, and then reassemble. Then just follow the installation instructions you got with the GPS. Good luck
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