I did that with my 1st Gen RX7 for my radar detector. I bought a 3.5 mm audio plug and jack, drilled a hole in the light lens for the jack and wired that up to the 12v line. Took the power right off the bulb hot points.
Then I cut the power cord to the desired length and connected it to the plug.
As long as you insert the plug with the GPS/detector off you will never short out the unit.
Sorry no pictures, sold that RX7 a long time ago and never took pictures of the install.