I posted this in the electrical forum but I don't think it's as exciting as this one.
I bought the under dash courtesy light kit from LMC for my 1978 F-250 Supercab. I don't seem to have the factory harness with the plugs like I see in some of the wiring diagrams. I would like to wire these into the existing dome light circuit so they turn on when the dome light turns on using the headlight switch as well as the existing factory door jam switches. Has anyone done this? I'd rather not add the additional door jam switches that come with the lights. If I understand correctly the green/yellow wire is always hot and the blue/black wire supplies power to the light after the switch. I should be able to tap into the switch wire (blue/black) for power and then ground the black wire? These lights look like the provided door jam switch grounds the circuit as opposed to the dome light which I don't think has a ground. Looking for some insight before I start splicing. Thanks!

 

Wire harness, underdash courtesy lights
D7TZ-13A709-A

Klimesh Motor Sales, Calmar, IA has 1 (563) 562-3241
Red Taylor Ford, Corning, AR has 2 (870) 857-3518
McLaughlin Motors, Sumter, SC has 1 (803) 773-1481

This is the factory harness.
You unplug the dome light, plug the courtesy harness into the main wiring harness, then you plug the dome light into the courtesy harness

If you do your own wiring, you're correct. Ignore the green/yellow wire and tap into the black/blue for the courtesy lights.

 

Thanks Mike. that single dome light in these super cabs leaves a lot of dark spaces in the front. I'll post back with the results in case anyone else chooses to put in those aftermarket lights.

 

Here is some old advice from FMC400 when I was working on my dome light issues. Maybe it will help?

"For the dome light wire, hot-at-all-times power comes goes to the headlight switch as a GREEN with YELLOW stripe wire. Dome light power comes back out as a BLACK with BLUE stripe wire. The same color scheme is used at the door jamb switches. BLACK with BLUE stripe runs out to the actual dome light. BLUE with a RED stripe is instrument panel illumination. It comes out of the headlight switch to the fuse box, leaves the fuse box again as the same color and is distributed throughout the dash.

There is no ground connection to the dome light switches or the headlight switches. These switches switch power, not ground.
1) Make sure you have 12 volts on both sides of the courtesy lamp fuse. This circuit is hot at all times, so it doesn't make a difference what position the key is in. If you have power on only one side, replace the fuse. If you don't have power on either side, then there is other work to do first.

2) Pull the dome light switches out and remove them. Make sure the terminal for the GREEN with YELLOW stripe wire in the connector has power.

3) Check the dome light switches for continuity. With the plunger all the way out (switch CLOSED), the switch should be shorted. With the plunger pushed all the way in (switch OPEN), the switch should be open. If not, replace the switch.

4) With power to the switch proven to be good, and the switches proven to be good, the dome light should light up. If the dome light does not light up, remove the cover and check for power at the dome light (not the side that's permanently grounded to sheetmetal) with the door OPEN. If you have power here, yet the bulb does not light up, replace the bulb.

This will prove out the door-side; if you get to the point that the doors turn the light on but the headlight switch will not, then we can cross that bridge."


You should be able to tap into the dome light power and get the under dash lights to work off of it.

 

Here's the illustration for the underdash lights.
In the bottom illustration that Rich posted, in the lower right corner, is the plug that the courtesy light harness patches into.
Unless you have the roof mounted cab lights you can ignore the 14677 relay that's located above the left courtesy light, it won't be there.

 

I would suggest you send the LMC stuff back. Watch ebay. For the same price you can get an original harness that will plug right into your existing set up. If your willing to check the local JY you could probably find a set for even less.

 

Thanks Mike for the diagram, I had it, but the PDF was hiding from me. lol

 

I added a led strip under the dash and it is amazing! Was like $20 at NAPA, tied into the courtesy lights. It sticks to the bottom of the dash and you cant see it at all.
These pictures don't do it justice because they were taken during the day but at night it is really bright.

 

Thanks for the help identifying the wires. I tapped into the blue/black wire for power and grounded the black wires. Lights up nicely!

 

installed a dual color led strip under my dash with a small toggle to swap colors. Pictures don't do really show how bright they really are.

 

Those LED strips are pretty cool. If you look under the front seats of a mid 80's Lincoln Town Car they have the same bulb holder as the dents to light up the rear footwells. Those would be a cool addition to a crew.

 

I understand this thread is a couple months old but thought I'd give this a shot..
79 Crew Cab - courtesy lights are the only lights not working. Replaced bulbs, nothing. Checked voltage and I have 12 volts grounded through the stock courtesy light fixture.
I can even shut off voltage with the door jamb switch(per the meter).
What am I missing? I even tried a different aftermarket light and same story.
How can I have 12 volts at the fixture but the light doesn't work - has to be a ground issue right?
I show a good ground with my meter though.
I can take the aftermarket light and the stock Ford courtesy light fixture up to the battery with some jumpers and they both work.

Is there anyway the headlight switch can be causing an issue here? It looks like those BK-BL wires go through the headlight switch before they get to the courtesy lights.

 

I doubt it.
Does your dome light work? This is also on the same circuit as the under dash lights.
If things are like you describe, it would just about have to be a ground issue. Are you having any other electrical issues?
I would check the continuity between the socket and something like the floor pan or firewall to see if it is a ground issue.
You might have to run a ground wire from the dash to the firewall.

 

I don't have a bulb for the dome light so not sure if it works or not - it has 12 volts though!
Everything else seems to be working correctly.
I agree about the ground being the likely suspect - but I can place the black cable from the meter into the hole in the bottom of the dash where the fixture mounts/grounds and the red cable into the courtesy light socket and 12 volts +/- shows.
Even checked Amperage and I show I have 2 amps at the end of the courtesy light pigtail which should be more than enough for a little bulb.
Never seen anything like it - I've been messing with this all afternoon.
I'll try to do a continuity test next I guess - grasping at straws at this point.

 

I just replaced my original dome light lens and bulb with the LED bulb and lens that LMC sells. Made a big difference!!