I just installed the OEM flat 4 wireing harness - it comes with a couple of relays that install in the pas. side cab bottom right - with the pig tail that plugs into the back with a nice mounting bracket. Recommend you get this one. Got mine at Courtesy Parts for $46.55 + shipping. You have to specify manual shift or automatic - the reqirement for the relays are different.
There is about a foot long harness for the relays.I'm not sure where it goes since mine came with the tow package.I did buy a 4 pin harness kit and tapped it into my 7 pin harness.It came with 3 relays,relay harness,traiker harness,mounting bracket and hardware.