I need help on this one. I was hauling my boat when I noticed the tail tights were not working on my 1999 F150 or trailer. When I disconnected the trailer the problem still existed. Then I noticed some other things, the dashboard light didn't work and the headlights wouldn't go off. I have to disconnect the battery to get the head lights to go off. Ahy Ideas?

 

First I would remove the headlamp switch and check the connectors for pins pushed out and contacting other pins.
It sounds like your taillamp circuit is open and you have a short to power causing your headlamps to stay on.

You might want to try disconnecting the headlamp switch to isolate which end, if any, is at fault.

By what you are describing,I would suspect the headlamp switch has a problem. Without being there it is hard for me to help you diagnose this one. But I will help in any way that I can.
Need some more help, just let me know..

Good Luck!

FordMasterTech

 

I suspect something in the switch also. Even more confusing is that the headlamps do go off and don't come back on when the battery is reconected. That makes me wonder if a relay is involved. If shorted to power, I think they would go back on when the battery is reconnected. Does the dash have to be disassembled to check this switch? And, how difficult is that?

 

Come to think of it there is a relay for trailer tow running lamps.
Try removing it and see if your problem disappears.
I would say that I doubt it will make a difference, but I have seen some crazy elecrical gremlins in my experiences.

One other thing to consider is that the multi function switch involves headlamps too. The only thing that it does it direct voltage for high or low beam & flash to pass function.

Let me know if I can be of more assistance.

FordMasterTech