Rear Junction Box

[AM0608]

I just bought a 2001 GMC Sierra 1500 5.3L. The 3rd brake light works and the driver's taillight works other than that no other rear lights work on it. I was looking into it and found out it could be the rear junction box as it seems to be a somewhat common problem. I looked up the rear junction box and saw pictures of it with 4 wires going to it. Mine only has 3 wires going to it. I can't seem to find a 4th wire anywhere on the truck that looks like it would go there.The wires that are going into it I traced and 1 is going to one side light, the 2nd is going to another side light and the last is going to a 7 pin plug. Am I missing a 4th wire that powers the rear junction box up? Or does it do something else? And where should it be wired from on the truck? All front lights work fine. Any information or help would be greatly appreciated, thanks.

[bphall1980]

Its probably the plug that is going to your tag lights, it also contains the ground wires for the turn signal flasher module on the back side of the fuse panel.