I just purchased a 1998 f150 and the third brake light doesn't work. The truck had a cap on it at one point and the wiring for it is still there. I pulled it out and there was no bulb so I put one in and it still did not work. I checked for power at the socket and with the key off there was nothing. key on it goes to 12v. press brake pedal and voltage does not change. I am guessing there is a wiring issue but not sure where to start looking any ideas would be helpful.

 

Have you checked the fuses?

Also, if it's got cruise control, be sure the cruise control recall has been taken care of.

 

Sounds like a wiring issue. It shouldn't have 12volts until the brake pedal is depressed. If it has 12 volts with the key on, does the light burn then? Try another bulb.

 

So here is what I have found. All fuses are good. No open recalls. Bulb is good. With the off there is no voltage to plug and it does not change when pedal pressed. With the key on the voltage goes to 10v and does not change with pedal depressed. When started I get 12v and does not change with pedal. Bulb never lights up when in socket but with multimeter I get the voltage reading. I am checking power at the plug with the bulb removed, red lead from meter to green wire and black lead to black wire.