rocketraider

Any of y'all happen to have a factory trailer wiring diagram for a 93 F150, 5.0L 4WD? From the power distribution box underhood on? Haven't been able to find one online and a buddy's AllData didn't have it either.

What started out as a headlight switch replacement has blossomed into a full-blown cluster. Apparently NAPA's replacement headlight switch is not exactly correct for this truck and I broke my own cardinal rule of not verifying terminal nomenclature before plugging it up. Terminals are labeled different from OE Ford switch even though they were all in the correct orientation for the plug.

Blew a 25A fuse feeding trailer harness soon as I landed the battery cable. Replaced it and it hasn't blown again, but with the NAPA switch, everything worked fine till I tried to turn the lights OFF. Park-tail would not turn off. Put a correct BWD brand switch from Advance in it and same results so I've got a backfeed somewhere, just don't know where.

Also replaced the headlight switch plug since the park-tail wire cavity was scorched. New connector had a couple of different color wires from factory wiring, but did that one wire at a time, matching the cavities in the new plug to existing body wiring in the same cavity as original plug, so I feel confident it's not that. Basic voltage and continuity tests verify plug wiring is correct and sound.

Based on that blown trailer fuse I think it's in that wiring somewhere, I just don't have a comprehensive wiring chart to know where to start. The fuse block diagrams in the owner manual are slightly clearer than mud, but apparently there's several fuses and a relay for trailer wiring, which I'm wondering if that incorrect switch caused that relay to stick creating a backfeed thru it. Would be great to know which one is the source feed. The AllData page stops at a brown wire "with trailer tow only" coming off a splice in the front lamp wiring harness, but does not indicate where to find the trailer harness diagram.

And natcherly all this happens when I'm stuck in a stretch of 80 hour weeks at work and absolutely don't have time to fool with it. The old gray wagon is down too and haven't had time to diagnose why it won't start, so I'm reduced to driving the only nice daily driver I own to work. If anything happens to the Mercury, I'm walking.