The wire you speak of, unless it is to ground you will not be able to access GPS functions or use the DVD function whilst you are driving. This safety issue is built into most units and its up to you if you wish to connect it, basically if you dont want the safety feature just ground the wire permantly or as the instructions, wire it to the park brake wire on the park brake switch (Red/white) found at base of park brake lever under the boot. Then the above functions will only work when you have the Park brake "On"