The spare supply behind the headlight is switched by the ignition key - if the ign. isn't on you won't have any 12v power. If you find that with the key on you still haven't got 12v at your GPS there must be a break somewhere in either your 12v feed or your earth. If you have 12v at the plug that goes in to your GPS but the GPS still isn't showing the 12v feed you may find something in the GPS set up menu that allows you to select the power source.