Got a 93 f250 5.8w 4x4. When the parking lights and head lights are off i have brake lights but when i turn the parking lights and head lights on, i dont have any tail lights or brake lights. And the cargo light gets really bright when the brake pedal is pressed. When i turn on the hazards the front ones work but the rears do not. Same with using the turn signal switch. I check all the fuses, theyre good. Havent had the trailer light relay in for years. The chime and dome light connectors have been disconnected for years. Not sure where to start looking.

 

So the rear parking lamps simply don't light up at all do you think you can feed 12v directly to both left and right parking lamps and see if they light up? Same for rear turn signal lamps, try that. Put that in other words is to bench test the rear bulbs, and don't trust yourself by looking at the bulb filaments.

So during daytime (when headlight/parking lamps are off) you have brake lamps working, but the cargo lamp (the interior side of your 3rd brake lamp just right behind correct?) also illuminate with the brake lamp at the same time. Correct? If that's true, how about your 3rd brake lamp on the outside when you press on the brake (during daytime), does it light up or not? If not, can you bench test the bulb too?

Just wanted to make sure no bad bulbs to begin with.
Let me know the answers above then I will attach relevant diagrams on my next reply.

 

Bench test good, turn and tail lights on both sides. Forgot to mention its a xl so it doesn't have a 3rd brake light, just the single white light.

 

C273 (Main Light Switch) below:

 

Locate C231 above and test all 3 power feeds (hazard, turn and stop).
Hazard for example:
1) voltmeter black to chassis ground, voltmeter red to circuit 385 (W/R), you should get 0V
2) turn key to ON/RUN position, you get 12V
If any of the 3 power feeds is not working, you can narrow down the issue. Please confirm all 3.

To see why your rear turn signal lamps are not working is straight forward since your fronts (circuit 2 and 3) are working but not your rears (circuit 5 and 9), so let's test right turn with key to ON/RUN position and turn signal stalk to up position:
1) voltmeter black to chassis ground, voltmeter red to circuit 2 (W/LB), you should get 12V because you said your fronts are working
2) now switch the voltmeter red to circuit 5 (O/LB), if you have 12V there then there is a poor connection somewhere perhaps at connector C205. If you don't have 12v then you know you have a bad turn signal switch, you can disassemble and fix or buy new.

Below are turn signal switch testing and inspection in more details. The instructions above are my quick test simplified version.








If you get 12v at C205 connector and C401, the last test is at the tail lamp, right tail lamp for instance, you test the O/LB wire for present of 12V. The chance you have broken wire at the tail lamp on BOTH left and right is low, but not impossible. It could be just corrosion at the connectors, if you look at C205 above both left and right go thru it, so the chance is higher.

Below is the index page of Lighting_and_Horns-Full_Electrical_Diagrams.pdf:



Color code below:

Four additional pdfs below.

 

Thank you. Will reply back if i find something .

 

This is a shot in the dark, but I am wondering about the ground for the rear lamps.
What I would try is to run a wire from the battery negative cable back to any lamp ground wire and see if it gets rid of the problem while you are providing a KNOWN good ground. I could be wrong, but I suspect the rear lamps are using.the cargo lamp for a ground and that would cause that lamp to illuminate.
We don't at this time know where the ground for the rear lamps is located, but if this solves the problem, we know we are on the right track.

 

What we know thus far, the front turn signals always work and rear turn signals never work, the turn signal issue doesn't depend on the parking or brake lamp. Based on this it's easiest to resolve the turn signal issue first that's why I enclosed just the turn signal diagrams above. The other issues take a bit more time to troubleshoot so I was lazy and enclosed full pdf files instead.