My son decided to get him a dent side like his old man. But i think its because he knows I can work on them. It is a 78 F150 that he has had the truck for about three months now and has already got the truck hot and I had to find heads for his motor and have them rebuild. I don't mind I will do anything for my kids. But now his headlights have quit working and I have changed the headlight switch and the dimmer switch. When I put a test light on the fuse it has no power but when I pull the headlight switch to parking light the fuse has power but as soon as I pull it all the way out it loses power across the fuse.

After I rebuilt his engine 351m

 

Sounds to me like you have a defective switch. Just because it's new doesn't always mean it will work, unfortunately. Do you have another switch you could try?

 

My headlights wouldent work and the tabs that hold the fuses were loose and not making a connection, I got a fuse box from the junkyard and that fixed the problem.

 

There are no fuses for headlights on these trucks, or any old Ford for that matter. You guys are not looking at what you think you're looking at. Circuit protection for the headlights is done by a breaker inside the headlight switch itself. The only fuse that touches the headlight switch in any way is for the dash lights. I'd like to know how replacing a fuse panel fixed a headlight problem because nothing related to the headlights goes through the fuse panel.

To the OP - you are probably looking at the dash light fuse which explains why it has no power with the switch all the way in, but does have power when you turn the parking lights on. The fact that it loses power again when you turn on the headlights likely means that there's a short in the headlight wiring under the hood, and the breaker inside the headlight switch is interrupting.

First confirm what fuse you're actually looking at. If I'm right and you're actually looking at the dash light fuse, then check what I said above. If not, then start over and remove the headlight switch. Check for power from the fat YELLOW wire inside the headlight switch connector. It should be independent of the ignition switch position (hot at all times). If that checks out, then you can move on to checking the OUTPUT of the switch.