OK so I have a 99 f150 5.4. When I turn on the head/Parking lights the #18 5amp fuse ( instrument illumination (dimmer switch power ) Blows?? Everything else operates just fine. I have tried this :
pulled after market radio and checked all wires,
pulled cluster checked bulbs,
pulled light switch unplugged dimmer... blows fuse
unplugged the middle plug.. turned on lights and light alarm works but does not blow fuse ( the head lamp does not work unplugged)
checked for frayed wires
any help out there

 

You are right 5amps. Reading the items this fuse controls, I'd bet on a bad headlight switch. I first thought park light relay, but you say the p/l's are operational. That leads me to the HL switch. If you have trailer towing harness, check that before replacing the head light switch. A bad trailer wire can cause the HL to go and blow the fuse. Shows being on the same circuit.

 

I forgot to mention that I unplugged the trailer brake harness from the controls at the dash. Shouldn't that stop the current for the harness. I've been thinking about the light switch and also the multi function controls. I'm going to test the light switch first. Remember. . The fuse only blows when I turn on the lights.

 

Unless you have a short in the wiring, then the headlight switch has gone TU.