I'm new to all this GPS malarkey myself, but just yesterday I mounted a Quest 2 on a 1200GS. I was full of trepidation but it was all very quick and easy. I routed the power cable behind the side panel.
(I need to sort out some options for routing the earphone cable. Probably getting an audio cable extension to go from unit to just in front of seat, so I can simply unplug earphone cable from the botton of my jacket when I get off the bike. Ideally, I'd like another socket at neck level so I can take my helmet off without unthreading cable from my jacket. Don't know if too many sockets along the way will interfere with quality of sound, though. There's a recent thread about earphones - I bought the cheap earphone from Maplins as suggested, but as they are dead cheap I'm going to buy a couple more and experiment with fitting them in my helmet. The sound quality through the earphone is much better than the tinny sound you get from the car speaker. and I had no difficulty hearing instructions up to 90 mph on my bike, even with moulded earplugs fitted.)
Mounting parts came from GPS World - click on the RAM mounts banner and that'll take you to it. The parts I ordered were:
RAM-B-149 (the mounting arm bit).
RAP-319 is the little adaptor shim thing to allow the mount to fit to the oval rail above the clocks (rather than to a round tube like the handlebars).
The holder itself is a RAM-HOL-GA15
The power and audio adaptor is called a QPAC04-xx (xx = jack socket size). This is the expensive bit at £40-odd
Don't take my word for it, though. Call the number on RAM site and get their professional view on what you need in case I've got the numbers wrong! Steve (RAM-man on this site) has been extremely helpful.
(Finally, to fit the mount you'll need an 11 mm spanner. And I cut a piece of rubber mesh to go between mounting bracket and rail to prevent any marking of the rail.)
Good luck!
Robin.
Wish I'd looked earlier. 
