i have a 99 f350 psd, my problem is i don't have parking and instrument lights, it blows fuse number 8 as soon as i turn the switch on. i checked all the wiring on my tail lights, and the trailer hook ups. i even disconnected the wiring harness going the my tail lights and trailer hookups and it still blows the switch. #8 fuse is trailer tow elcetronic brake, illumination, park lamps, trailer tow park lamp relay coil. it also blows #4 fuse which is trailer tow backup lamps, trailer tow park lamps. help someone!!!

 

Get a wiring diagram and start backtracking.

 

where can i get a wiring diagram?

 

The complete wiring diagram will be included in the service manual. If you don't have a set some dealerships will make a copy of the pages you will need for troubleshooting.

If the fuse blows as soon as the switch is turned on it is very possible that you have a short circuit. Have you tested any of the wiring for shorts to ground?

 

Sounds like there is a short to ground somewhere between the switch and the relay.

 

where us the relay located

 

sorry, i have no idea where it is, i am just guessing based on your symptoms. you will need to consult a '99 shop manual for the electrical diagrams. I got one of those ebay shop manual CD's for my 2003 which works pretty good. Don't have it with me now though, or i would check for you.

 

I'm not familiar with the 1999, however normally relays are in two locations - fuse box in the cab and power distribution box under the hood.

An abbreviated electrical schematic can be found at this link:
https://www.fleet.ford.com/truckbbas/topics/1999/append_wiring.html

Owner's manual can be found at this link:
https://myford.fordvehicles.com/servlet/ContentServer?pagename=Owner/Page/HomePage

 

I'm having this exact same issue. Did you ever figure out what the problem was?

Mine started as intermittent blowing of the fuse and after a while it just went out completely and blows shortly after replacing. The parking lights don't work at all and the dash lighting holds for about a minute after which the fuse blows.

Very frustrating as I'm forced to drive with hazards on at night.