Personally I think the best place to mount a GPS is on the cross bar above the handlebars by the screen. There's a pic of as R1200GS with GPS on this bar
here, and visible if you click "360 spin" link.
Mounting the GPS to the left of the bar doesnt obscure the instruments, and is more in the direct line of site with the road (less eye movement). I'm not a fan of handlebar mounted GPS as IMHO it gets in the way.
As for wiring it in, there are 2 ways that I am aware of.
1. There's a BMW power connector near the horn, and you need a special lead to use this (or you could chop the connector off).
Nippy Norman sells a lead to allow you to use this connector. This connector is wired to the CANBUS electrical system of the bike and is only live when the ignition is on, and possibly for a few minutes afterwards. I think the CANBUS can supply max ~5A, but then a GPS is going to consume mA so this limitation is not relevant.
2. My preferred way - direct to the battery via a fused connection. Running a lead from the battery under the seat to the screen is easy. You don't need to take the tank off (although it does make for a neater job). Pull off the small black plastic panel on the LHS beneath the seat, squat down and look inside the bike towards the front, but undereath the fuel tank. You can see part of the front telelever suspension running towards the back of the bike, and some cables / pipework funning front to back, and daylight at the front of the bike. You can run a cable along here, and zip tie to whatever is convenient. Just be careful of the wire rubbing on anything. Once at the front, just guide the wire up to where you want it and zip tie etc. At the battery end, probably best to use a Centech Fuse panel (
Nippy Norman), and locate underneath the tool tray under the seat.
Took me 20mins to wire in my GPS on my new GSA last week; wiring is the same with a GS; but then its my 3rd GS.
Other Info
I have a Wunderlich handlebar bag (
Nippy Norman), and also have a cable from the battery/Centech fuse panel terminated here with a regular cigarrette lighter socket on it (Maplin). Whilst the bag isn't claimed to be waterproof, it is, and use it to charge phone / ipod whilst riding.
I'm in W4, you are welcome to come have a look.