Medium and Heavy Trucks
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Hi my name is***** will try to help. What is the year of your truck so I can see if I have any schematics that will help?
Thanks for the info. I do not have a schematic for that year unfortunately but you can purchase manuals and schematics here. http://www.faxonautoliterature.com/Service-Manuals/Ford/LN
what you want to do is with the IGN and light switch on, do a wiggle/pull test on the harness to try to manipulate the harness to make a connection for the lights to come on. If successful you have found a poor or broken connection in the circuit. If not you can test the wiring and switches with an OHM meter to determine if you have a faulty switch or wiring. Is the dimmer switch in the turn signal switch on your truck or on the floor? lost both high and low beem?