Over the years I've collected a bunch of car cell phone adapters. I found one that had an inline fuse so I pulled it apart and cut the Garmin power cord the length I needed it. I then soldered the cable onto the power contacts of the plug. I used some silicone sealant to hold the cable in place on the plastic housing so it couldn't be pulled out. Most phone chargers have a 5 volt output. So you can remove the circuit board used to regulate the voltage. In my case, this adapter had a red LED to show when power was on, so I kept it in the plastic shell just to light the LED.
So when I need the GPS, I just attach the unit to the ball mounted on my clutch bracket and velcro the wire to the handle bar and plug it into the cig lighter. I came up with this because I was leaving on a trip and didn't have time to pull the fairing off just to find power. I like this because there's no wires left behind when not using the GPS.
Ignore the red heat shrink, damn cat.