this may help, look under your dash and look to see if anyone has ever hooked any wires up to your brakeswitch wire or the 12v going to that plug. the plug is located right above the brake pedal.. this can be a common problem if anyone has ever install a remote start system on your truck seeing how it uses that wire to kill the remote start when the brake is depressed. also check behind the dash where your dimmer switch is and your ing harness.. someone may have tapped into your light switch for a parking light wire for an alarm as well.. good luck