Look for a ground block behind the dash. A lot of times manufacturers will run a positive cable and a negative cable all the way to the dash area to tie into. Sometimes you wont find a positive block. If not you can either wire it hot to one of the switches, or if you have a unused switch or location for a switch tie it into that.
Don't tie it into an existing switch on the switched side. A Gps probably doesn't draw that much but you could over load the switch.