I just replaced my third brake light. Old one was broken. Went with an LED replacement. First thing I did was test it with a bench power supply. Works as it should. I forgot what voltage range I tested it at. I think I started at 14V and lowered the voltage to about 8 or 9 volts before it cut off. Anyway, I KNOW the red and white light both work as intended. I had to cut the connector off the truck a long time ago. I don't recall why. Doesn't matter. So, I cut the connector off the light, too. Using spade connectors to hook the light to the truck. I have it wired up correctly, but I cannot get either light to work. It started to rain, so I had to call it a day, but I was checking the wiring with a DMM and I'm only getting about 8-9VDC from the black/lt blue wire (cab light) to the black wire (ground). Wasn't able to test the brake light before it started raining. I did check the lt green (brake) wire to the black wire (ground) withOUT depressing the brake pedal, while the cab light was supposed to be on and I was getting something like 800mV... What gives? Messed up relay, fuse, melted wire? Where should I start?

I forgot to mention - 99 F150 XLT reg cab. When I was checking the voltages, I had the truck running. Over 14V everywhere else BUT the CHMSL.

 

After thinking about it more, I'm pretty sure it's a bad ground. Any clue where the ground is terminated on the body for the third brake light?

 

Here is the wiring diagram. The ground wire is black. I don't know where the G200 ground point is located. There is a splice at S911 and a connector before the wire reaches G200.

 

I ended up making a temporary ground wire for the CHMSL and screwed it into the cab behind the light. Not ideal, but the light works fine. I'll pull the headliner down and fix the ground wire properly when the weather cools off some. That light doesn't draw hardly any current, so I'm not worried about it.

Thanks for the wiring diagram.