4, 5, 6, and 7 I've seen; never seen a 9 pin. If you'll go to google.com and enter "trailer light plugs" and "trailer light connectors" you'll find more than you want to read on the topic. The 7 pin connectors that I'm familiar with are: (1) ground, (2) taillights, (3) left flasher or turn signal, (4) right flasher or turn signal - the flashers are also the brake lights usually, (5) electric brakes, (6) 12v continuous - to charge a battery or operate a 12v refrigerator or lights in the trailer, and (7) auxilliary - sometimes backup lights on the trailer.
If it were me, I'd want the 7 pin connector on the truck, and then there are plenty of adapters on the market to go down to whatever circuits are needed for each trailer, or of course, you could put 7 pin connectors on all your trailers but only have the circuits needed wired into the trailer plugs.
Bird has it right, in my opinion. Convert the truck to 7 pin with all the necessary wiring and use the adapters locally available to convert it to 4 wire flat. Convert the other trailers to 4 pin as well.
Bird,
You are sooooo right! I wired my old truck up 7-wire rv round. Took the time to do brakes, since I towed the tractor on a trailer. Had backup lights for the horse trailer. The newer Dodge was pre-wired 7-wire from the factory.
I set the horse trailer up 7-wire. Set my old slide-in camper 7-wire. My new truck, and flatbed trailer are 7-wire. Every possible accessory works.
I have a 7-wire to 4-flat adapter. I also have some pigtailes so I can adapt to 4-wire round, and 6-wire round. I can pretty much adapt to anything; pretty easily too.
I have found it is easier to adapt down than up. It's hard to adapt up from a 4-wire flat to a 6-wire round.
I am pretty well set on my own "towables", so adapting usually isnt necessary. And, on all my "towables", all the options are on them for lights, brakes, and charging.
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