I wired in a 12v cigarette lighter/accessory socket (£3) with an in line fuse to the battery. It's located under the saddle. I routed the Garmin in car power lead so that the plug is at the GPS mount and the power plug is under the saddle to plug into the 12v socket.
This arrangement means that you have another accessory socket to plug other things into, that can be locked under the seat. I use mine to charge my phone securely while setting up camp.
The socket is always live, so don't leave any thing plugged in when the bike is not running for too long.
I'd rather have a bottle infront of me, than a frontal labotomy.