Just to update this thread, and I have moved it to the electrical archives.
My 82 Cherokee came with the trailer towing package, and a 7 pin factory trailer plug.
As far as I can tell, when you have the trailer package with the little black box of 4 relays, there is no way to just add a 4-pin flat connector. The lights will work, the turn signals will work, but brake lights on the trailer won't light.
You need to either go thru the factory 7 pin plug using a flat 4-pin adaptor, or you have to add relays like Tony mentioned. I went thru my box and made sure all the relays worked, and used an adaptor.
You must have the 12 gauge bullet connectors, one in and one out, hooked to a hot source. Otherwise, you will hear the relays working, but nothing will come out of the 7 pin plug…. (that was my trouble)