Originally Posted by ala_xfire

You can splice in a female cigarette lighter socket to the glove box light ( it's common to the cigarette lighter socket in the console )
and plug the GPS cord into that. ( shell to ground, center contact to 12V )
It will come on with the key, and go off when the ignition is turned off.
That way, you don't need to modify the standard GPS power cord.
Hide it all up behind the glove box.