These instructions are unofficial and I just show how I wired a Garmin Zumo XT to a Ducati DesertX. I guess that the process is same for any other GPS unit.
And sorry for blurry photos. I wasn't focused on photography.
1. Remove the windscreen and plastic cover. If in doubt, see paragraphs 1 and 2 HERE.
2. Locate these two. One connector has a rubber cap and it is tied to the other connector with a rubber band.
2. Remove the rubber band and disconnect this one.
4. Remove the rubber protection piece. Be careful not to cut the wires underneath. I used the lighter to heat it a bit and it slid off, leaving the two wires exposed.
5. Remove the isolation from the wires. Be careful as the wires are short.
6. Connect this to the wiring that came with your GPS - wire to wire connection. Try it on your bike before soldering it - isolate the wires (!!!) than connect it to the bike and see if GPS unit has the power (with key turned on).
7. Solder it...
8. And isolate properly.
9. You should have something like this now.
10. Ready to connect to the bike.
11. Tidy up the wires and re-install plastic cover and windscreen.