Originally Posted by Mmnsc
Truck is a 2002. As soon as I get a chance I will have to take the truck to a Ford dealer to do a check of the electrical system. This has me really stumped.
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After reading through and you claim there is 9v @ the taillight bulb terminal I'm suspecting bad headlight switch. Also if during the testing you only had 9 volt at the chassis harness then you need to keep looking at the harness until it enters the cab. If all looks well, next I'd check the headlight switch. Over time the contacts in the switch get dry and form a film of corrosion causing resistance which will show as low voltage with everything on.
If you want to double check the trailer plug, lay under the truck look up at the plug. The harness will unplug @ the backside of the 7 blade plug. Then follow the harness towards the driver's side frame rail. There you should find another plug. Unplug BOTH of these and inspect the harness and the teminals inside. If they're green go to the dealer and get a new one.
While it's apart, use a meter and see if you have full battery voltage at one of the terminals with the parking lights on. If you do have good voltage now then the problem lies in that pigtail you removed.
An older truck like that and without seeing it myself, I'd suggest replacing the plug and pigtail harness.