So after realizing all my rear light sockets were bad off, I decided to just go with replacing the entire rear wiring harness. Picked up one from mr. taillight, looks great. Pulled out old harness and replaced with new one, got all sockets and bulbs in and connected, screwed in ground connectors that go in behind the tail lights. Then I went to splice the main wires from new harness in to factory wires coming from the front. Problem. There are the 4 "hot" wires coming from front, but the wiring harness has 5 wires because of the black ground. Where does it connect to? I understand it ends up behind the tail lenses, but where does it start from? Is there supposed to be a ground wire that runs from the front with those other 4? Any help is greatly appreciated. Thanks!

 

They do it 2 different ways depending on the vehicle, either a ground wire in the harness, or a seperate ground wire to the frame near the back. If you need to add your own just do a real nice job cleaning the frame, and use lock washers to cut into the terminal, then paint it up nice.

 

Each socket's ground wire is just ran to the sheetmetal behind the tail light. You will see a small screw hole behind the tail light for it. Clean it up, ad an eye connector and screw it in. Same screw hole is used bu the marker lights too and I think the reverse light socket. Running a separate ground wire doesn't hurt if you want to go that route, bit a good clean sheet metal ground works fine like factory.

 

Yeah, the factory ground location is a sheetmetal screw behind the taillight assy.
The bed itself grounds to the frame by contact. If you're ambitious, it wouldn't hurt to add an actual ground cable between the bed and frame.

 

So I have those ground wires screwed in behind the taillights, but where do those grounds start? The wire harness I purchased STARTS with a ground wire in addition to the 4 main wires. This is where the rear harness starts from the 4 wires running down the frame rail. Do I just start off those grounds on the frame somewhere?

 

where it starts, just tie it to the frame. There is no separate ground wire coming from the main harness running down the frame rail.

 

Huh, the harness definitely starts out as 5 wires, one of which is a ground. So just attach that to frame somewhere?

 

Not exactly sure on why they wired it like that without seeing how they did it out of the loom, but yeah, just tie it to the frame.

 

So if that random extra ground wire isn't that important, I have a new question. Once I installed new rear harness, my brake/turn lights worked along with reverse lights. Cool. But license plate lamps, one side marker and one tail marker do not work. Thought it was because of that ground not hooked. Just weird that some of the constants work and some don't. Again, new harness and bulbs. Everything spliced in nicely. Any direction would be greatly appreciated! Thanks again!

 

All of those loads are fed by the BROWN wire. You simply have to get behind there and inspect; there's not really any specific direction anyone over the internet can give you there. Since some markers work, you know power is getting back there in the first place. Also note that 194-style marker sockets are pretty susceptible to the elements.

 

Totally understand. Just to clarify though, all sockets, bulbs and wires are brand spanking new. Also inspected brown wire coming down frame, looks fine. Maybe the harness they sent has some issues. Left side marker works, but left tail marker doesn't. Right side marker doesn't work, but right tail does. Definitely not one side or the other. Confusing. Also plate lamps don't work either.

 

I would assume that all the ground wires from the sockets in your new harness are tied together in their harness. You would need to ground the black wire to frame.

 

Sounds real possible! I'll try that out in the next couple of days (supposed to be getting snow tonight) and see what that does. Thanks again!

 

I wouldn't say that the ground wire from your new harnes isn't that important because it's there for a reason. My guess is that (for looking at Mr tail lights harness pics), is the pig tail ground wire screwed behind tail light is for that light socket and the rest if the sockets are tied to that 5th black wire which you say is the ground coming from the start of your harness. Bad grounds can cause some strange things.

 

Since Mr. Taillight did all the pre-wiring of the sockets I don't know if it's possible the harness isn't wired correctly. Wouldn't hurt to give them a call. Make sure their color wires correspond with fords coming from the front of truck.