I can vouch for how little a GPS draws. I run a Lowrance Globalmap 100 unit which requires 4 AA batteries, and draws significantly more juice than more common GPS units. Even so, I regularly leave the unit powered on for days at a time (while in storage at home, NOT on the trail!) and have never had a low battery problem. The draw is very minimal. As for where I tapped in to, for both the GPS and my heated shield plug I just ran the wires to the battery jumper leads. I mounted a Radio Shack accessory plug on the right side frame rail under the hood. I mounted it there because my GPS wiring has a LOT of extra wiring that I wanted to hide.
Jim