I recently acquired a 1975 F250, 2WD, crew cab, automatic (FMX), 302cid. Both turn signals worked but were very slow to flash (approx. 4 seconds between flash). The flash relay on the drivers side behind the dash wasn't making a noticeable audible noise but when I touched it I could feel it operating. I replaced the flash relay with a NEW one. After turning the left turn signal on again, not only did I hear the flash relay, I heard another relay operating in the engine compartment, and the left turn signals were flashing approximately every second. When I located the auxiliary relay in the engine compartment it was mounted on the drivers side, just behind and above the ignition module. To my surprise I found a second auxiliary relay mounted beside the one that was operating. Every time the operating relay clicked the left turn signals flashed. When I switched to the right turn signal the flash relay in the cab sounded but the second auxilary relay was not operating and I had no right turn signals flashing. I am thinking these two auxiliary relays must be part of a towing package in order to accommodate more lamps. Unfortunately I cannot find any information on the wiring schematics however both auxiliary relays are connected with what appears to be factory three blade connectors.
Does anyone know if Ford had a tow package that included this kind of installation? If anyone has a wiring diagram or schematic, or knows where to locate one I would appreciate hearing from you.
Thanks,
Big Big Blue

 

Go here: http://www.fordification.net/tech/im...aster_7of8.jpg for the factory wiring diagrams. This is a '74 but the '75 should be the same.
The trailer package relays are to the right of the page.
To see all the schematics, go here: 1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net