1.2.3 Rear-View Power Socket
A GPS UNIT IS A GREAT CAR ACCESSORY, but the power cord hanging across the dashboard is an eyesore. Many vehicles have 12V power at the rear-view mirror, with a 2-wire plug. Tap into it to straighten up your view.
1. Wire your socket.
Unscrew the Philmore lighter socket to reveal the lugs. Strip ¼" of insulation off all ends of your wire pair. Solder one wire to the center lug (+), and the other wire to one of the outer lugs (–). Reassemble the socket.
2. Check polarity of the mirror plug wires.
Make sure your car is turned off. Use your ohmmeter to find continuity between a ground (such as the metal housing of the 12V socket in your dash) and one of the existing wires at the 12V mirror plug. The wire with zero resistance is the ground (–).
3. Connect your socket.
Push your socket wires into the mirror plug, (–) to (–) and (+) to (+), making sure they make good contact. Zip-tie them to the plug wires so they stay put. (For a permanent connection, you could solder them.) Finally, zip-tie your new 12V socket securely in its hiding place. » Your new 12V socket behind the mirror is ready to power your GPS navigation unit, radar detector, or other gadgets that are suction-cupped to the windshield.
NOTE: If your mirror lacks a plug, try tapping into a courtesy light near the mirror.
Going Further
To mount a gadget permanently, you could solder its wires to the plug, with no socket. But I wanted my GPS to be removable. 
Doug Watson ([email protected]) is a fixer of all things broken; sometimes they’re even improved. He recently turned 52, and his brother thinks he's finally playing with a full deck.