License Plate/Third Brake Light Wiring Woes.

After all these years I find myself in a situation where I need some help from you wiring gurus. I have a H/C rear bumper and bed cage that obscures the 3rd brake light. I have been running a light the last 12 years that mounts to the license plate that illuminates the plate when the running lights come on and also acts as a 3rd brake light. Well that light finally took a shit and I bought a replacement. It's basically the same thing but from a different manufacturer (Both have three leads).

This is the new light, pretty much identical to the old one which I can't find a picture of, assume it's out of production after 12 years.






The leads on the old light were wired into the following wires.

One wire went to a solid blue wire that was in a harness of 4 wires that led to the right rear tail light assembly. I assumed this was the brake light.



the other two wires went to a harness of two wires, one is white with a black stripe, the other is green with grey marks. If I remember correctly this two wire harness was what went to the license plate light.




I assumed blue was brake, White/black was ground, and green/grey was license plate. After hooking it up I immediately blew the tail light fuse. Using a multi-meter with all lights and the brake depressed I'm not getting a reading from any one of the three wires. Black probe to frame ground, red probe to each of the wires. I went into "Just touch the wires together until it works mode" but am quickly running out of fuses and one of the combos burned out the tail light(engine bay) and running light (cab) fuses.

I know its me...but help would be appreciated.

The leads on the new light are Blue / Black / Red.

 

Blue should be Brake Light
Green should be Tail Light
White/Black should be Ground

If you want to be sure.

An easy way to test this is with a voltmeter.
Check all 3 with the truck off and headlights off.
Set it to 12v. Take the black lead and touch it to the frame.
Then take the Red lead and place it each one of the wires.
With the truck off and head light switch off you should have no power to any of the wires.

Then, turn the head lights on.
Ignition needs to be On.
Find the wire that is now showing 12v.
This should be the tail light wire.
Turn the headlights off.

Next, press the brake pedal. (Keep it pressed)
Find the wire that now has 12v. This should be the Brake Light wire.

Now you should be left with one wire that never had 12v.
This should be the ground. Set the meter to ohm/continuity.
Red lead on the wire. Black on the frame.
See if it has continuity to the frame?

As for the new light.
All I can do is make an assumption.

If I were guessing, I’d say
Red is Tail Light
Blue is Brake
Black is Ground

Again, that’s a total guess.

 

Come on back to NC...Bragg misses you.

I would highly recommend investing in a Power Probe my friend. It can apply power or ground to a circuit without damage since it has a built in resettable breaker. It has saved me a lot time and money with all of the BS I have done.

 

Thanks for the feedback, I appreciate it and will give that a try tomorrow after work

 

power probe on the way, care of Jeff Bezos