When I first got my truck the tail light harness was totally wrecked and I had to make do with what I had and got it working.
Over the last couple months I have collected almost a full harness from the firewall to the taillights. Here's where it gets weird!
I pulled a harness from a 67 F250 that was, of course, hacked a little bit, but 90% complete due to a 'Y' in it that leaves a mystery.
The one branch of the 'Y' is the complete tail light harness, Brn, Blk, Yel, Lt. Grn and I have the whole thing right up to the tail lights.

The other part of the 'Y' is the mystery,

Is this a "factory" trailer harness? What does the Red wire plug into at the firewall? What does it go to at the rear? Is the White wire ground like I am assuming? The Brown is "lights", Green is "right brake and turn" and Yellow is "left brake and turn" correct? The 67 wiring schematic doesn't show a White wire going to the rear, or a Red either.
Anyone know what is going on with this?
Thanks!