Was really impressed with myself today when I wired my 3rd brake light up to the spare tire carrier then when I turned in the head lights tonight the 3rd light stops working.
Head lights off= 3rd brake light works
Head lights on = 3rd brake light doesn't work.

When I look at the wiring to the brake light bulb I have a blue (hot) for brakes and a white with brown stripe for ground and a black ground to the outer housing.

When I wired it today blue to blue and brown to white/brown it worked great until I turned on the head lights.

With the head lights on I can't get the 3rd to work at all.

Help !

 

Try running the white/brown ground wire directly to a metal chassis ground and see what happens. I think you're grounded to something that has power with the headlights on.

 

I ran the brown wire (from the light) straight to a good ground and then played "whack-a-mole" with the blue wire (from the light) and still got nothing.

It's bizare, I'm sitting there staring at the bulb which is lit with the brake on and my test light lights on the white/brown and on the blue.
I hook the blue from the light to the blue from the bold and ground the brown from the light and still nothing.

 

One end of the test light is grounded, it lights up when you touch the probe to the Blue AND the white/brown?
If that's correct then your chassis ground is floating above ground. There shouldn't be any voltage on the white/brown ground wire.
Try running a ground wire all the way back to the battery (-) and use that for test purposes. See if things work properly.

 

Ran a clean ground and still no luck. 3rd brake light still only works with the headlights off. I checked the wiring for the original 3rd and it didn't work at all so I grounded the new 3rd separately AND STILL NOTHING ! This is driving me nuts !

 

The only other possibility then is that the positive wire (blue) isn't attached to the correct wire for the brake lights.