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 auxiliary 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

 

4 way flasher is the 2nd one

 

Yes, Ford had a tow package that included aux relays under the hood for the trailer plug lights.

 

1973-1979 Ford Truck Wiring Diagrams & Schematics - FORDification.net


They don not have 75 up, but 74 and 76 look pretty much the same for the trailer relay set up.

 

I just found out about this today, was planning on making a system just like this for my truck, takes the load off the factory wiring to power trailer lights. But I would love to find one of these factory harnesses. It would be great if you could take some pics.

 

For the auxiliary relays I am pretty sure the Ford part number is D3AB-14677. I have priced them with a high price of $67 each, but I found NAPA has them as there part # MPE AR 200SB for $14 each. I will try to get some pictures but it may take a week or two as I will be gone and won't be able to get to it.
Thanks to all who have responded for your effort and information. I may have a few more questions as I get into this.
Big Big Blue