I have a 2004 F-350 diesel truck that I bought new. I have added two items that are in the tail light circuit; clearance cab lights and a rear stop/tail lightbar. 5 or 6 months ago I noticed the tail lights were not working. I checked the fuse and it was blown. I changed out the fuse (10 amp) and turned on the lights. The fuse blew immediately. I removed the headliner from the cab and checked the wiring to the cab lights. It is perfect and not nicked or cut or chaffed. I used the factory harness when I put in the cab lights. I did the fuse thing again and it still blew. I remover the rear light bar and checked it again, and it still blows the fuse. I have crawled under the truck and looked for animal chewing marks, cuts and shorts; nothing. I am not a hack when it comes to wiring, I can build computers and radios, using the radio shack kits of the old days (wired and no circuit boards) I have wired several Jeeps in the past, so I know something about how vehicles are wired. DOes anyone here have any ideas? Thanks.