I realize I should accept my guess work on this, but I have nothing to fall back onto.

My '04 F-150 has the Tow Package. This gives me a circular electric plug at the rear bumper. I know the circular use and schematic. Attached to the immediate Driver's side of the circular plug is a vertical wiring plug with 4 connections. This vertical plug has 3 female connectors with a male connector at the bottom. What is this vertical plug for?

I have my owners manual and a Helms Maintenace manual. Nothing is mentioned about this second, vertical plug.

 

The circular one is a 7 pin connector, the other is a 4 pin connector. If you do not have a trailer with trailer brakes, like on a boat set up, you use the 4 pin connector. Basically it is set up for your lights, running and brake/signal.
Edit: At the top of this page is a tester for the truck circuits. The plug that goes into the truck connection looks exactly like what would be hooked to the trailer.

http://www.shop.com/amos/cc/main/cat...Circuit-Tester

 

Thanks King for the advisory and link. On my past Tow Packages there was just the circular plug. I had the imagination that the current vertical, 4-pin connector might be for trailer brakes or special electric on large vacation trailers.

 

I tested the vertical trailer plug and it is a 4-pin trailer electric light plug.

Pattern:
Bottom male: Ground. Next up: Straight lights. Next up: Left turn/stop. Next up (top): Right turn/stop.

For a cheap wiring tracer here is my home made version that allows seeing the chosen pin function while standing at the cab:

Get a yellow (for good vision) 12v auto bulb. Solder 12' of 16ga auto wire to the botton lead connector on the bulb. Now wrap a few turns of stripped wire around the neck of the bulb and solder (or tape) the wire. Place small (for cliping on and sticking in pin holes) aligator clips to opposite end of both wires. Place a small piece of 1/8" screen over the bulb to allow protection (like a drop light). When you do trailer connector testing place the yellow bulb on the ground/deck near the cab and work your lights from the cab. You can see the yellow bulb light when you switch to a positive circut.

If you have only the trailer and have a 12v battery charger, use the charger to give power to the trailer plug. This cuts down on hooking to a vehicle for power and walking to the cab for each pin to be tested.