[Lurayfarmer]

Question for trailering experts. My "new" '72 Sierra Grande has factory-installed trailer wiring. The rear plug has the 7 round pins and I need to connect it to a 4-pin trailer connector. No problem, it would seem - these adapters are sold everywhere (as in Tractor Supply). The problem comes in because one of the round pins in the truck's plug (the top pin) is much larger than the others and will not go into the adapters. What's up with that big pin? Power for the trailer brakes? Is there an adapter that will fit or do I need to replace the truck's plug assembly?