Hi everyone, I have 2001 ford ranger 2wd 2.5l Lima motor I bought the truck a year ago and love it. It was bought new in Oklahoma originally and I bought it from the guy when he moved to ohio I've been on this forum when I redid my head gasket and some other things and It has really helped me out. RonD in particular lol. Anyways my third brake light does not work. It does not work if I hit the brakes and the outer white lights do not light up for when the doors are open. Heres the kicker, I was washing the truck getting the snow off of it a few weeks ago and I noticed the white lights were on, I was amazed, I went inside the cab and the dome light switch was all the way up to where the dome light kicked on and thats why they lit up, apparently that controls the white lights? Anyways my dome light works normal, as it should when you open the doors and what not. This really bugs me for some reason so any help is appreciated.
Thanks Brandon

 

Rear exterior Cab brake light and "Cargo Light" probably share the same Ground wire.

I would check that first, remove the lens and bulbs then test for ground with OHM meter, and jumper wire to Battery Negative.
200ohm scale, should be 0 ohms

Then switch Meter to DC Volts and put something on the brake pedal to activate the lights, make sure rear brake lights are on then test for volts at cab brake light.

Cab brake light has it's own wire to brake pedal switch, tail light brake lights can run thru turn signal switch depending on model year, but in any case they use different wire out of brake light switch than the cab brake light.

You can remove the cab light assembly but will probably need a new gasket for it, so have that available.
These lights are known for leaking, while not common, they do leak so lights and wiring can get corroded causing poor connections.

Google: ranger third brake light gasket