My 86 has a 9 pin also, to convert to a 7 pin just do not use the wires that go to the 12V
fridge (used with a 3 way fridge) and the teleprompter thing for the low tire circuit. That's the rod like thing between the axles, has a micro switch or some other vodoo operated thing that when a tire looses air pressure the rod changes length and activates the circuit and the indicator light located in the tow vehicle. I'm out the door right now so if no one else chimes in I'll provide more info later if needed.
as I recall there is also a 9 pin still available BUT it doesn't match up to the older plug on the drainpipe. My burb has the 9 pin on it so I made up an adaptor cable to a 7 pin, you'r just going in the other direction , ay.