Hello,

I just installed a cap for my truck. I ran a wire from the blue and black wire and yellow and green from under the hood that are meant for 3rd brake light and dome. I grounded on the frame under the spare tire. The dome still doesn't work and the 3rd light stays on even after I shut off the truck and lock it. Only thing thst shuts it off is unplugging my cap harness. I assume ground is good since the light comes on. Any ideas?

Thanks

 

What model and year truck you working on?

 

2013 fx4

 

So I'm looking at a 2013 manual which usually matches the 2014 .
I may be missing something, but I can't pick out on the prints where you tapped in,
the wire colors don't match (yet).
A few questions.
What fuse number under the hood are you using?
You say high mount stop light and dome light, are you tapping into the high mount lamp to use the exterior cargo light or are you trying to use an interior dome light circuit?

 

Cap came with a harness that has interior dome and 3rd brake light. Old owner said he tested both and they worked. I ran a 16/2 wire from under the hood to the back up through drivers taillight into the harness. Picked up a ground from under spare tire. From a search on these forums I found a picture and post that said there is a blue with black strip capped wire, and a green with yellow capped wire and they are the dedicated circuits for dome and 3rd brake. I found them right by drivers firewall near master cylinder. Dome doesn't work but 3rd brake comes on and stays on even after truck is locked and other lights go off. Note, my original ground wasn't good and neither lights worked but my tail lights stayed on non stop and would go out if I unplugged the harness. I assume that's the trucks way of telling me I had a bad connection. Not sure which fuse they are connected to. Harness has a fuse in each supply but I checked and they are fine.

 

So I looked through the manual again and I can't find those two wires of those colors anywhere.
Maybe I missed them, but I guess I'm no help.
Keep in mind that sometimes when you have a lamp that acts weird, (not powering off) you may be back feeding the lamp through the ground.
I think I would put a test light at the wires under the hood, one at a time and see if you get the results that you are expecting.

 

Here is a picture of the two wires I'm talking about. I will try the tester. Thanks

 

OK so what color wire is the yellow with green stripe spliced to on the truck harness?
Also the same question for the brake wire.
Maybe I can trace them back to the fuse/ relay panel.

 

Same thread says fuse 4 and 31 for those 2 wires. I haven't checked them yet but 3rd brake does come on so fuse must be ok

 

So, I pulled up the prints for the fuses. F4 and F31 at the BCM. Also, F31 at the battery junction box.
I still don't see the blue with black stripe, but maybe you can spot it.
As you said the F4 print does reference the wire being near connector C127.
Maybe this will help you.