I have a 88 XJ Pioneer and went through this drill in 2000. I also pulled off the spare tire mount and the interior panel to find just the harness, no connector for the trailer harness.
I went to the local dealer and asked about chassis harnesses. This was a "new" jeep dealer who never sold an AMC Jeep Cherokee. They parts guy looked up in the book and said they showed two chassis harnesses for the 88 XJ and the difference was the trailer wiring.
So you, like me, have a non-trailer harness XJ.
I found that NAPA is the ONLY place that sells a plug in harness assy, the part number is BK 7551577. That is the part that works on a 88 XJ, I just checked my orders from the napa on line site to confirm. It runs about $25. It a unit which uses vehicle power from the brake and turn signal circuits. You remove both taillights and plug in the trailer harness kit.
I had already purchased a "converter" which draws power from the battery instead of the tail light circuit. I then cut off the "tee" fitting and spliced it onto the leads from the I cut off the NAPA assy and it plugged and played. I had previously wired up this independently powered box to the tail light harness and to the right turn signal wire in the chassis harness in the driver's rear quarter panel. This is where my $20 88 XJ electrical troubleshooting manual paid for itself. I connected the right turn signal wires to the chassis harness insead of tapping off at the right tail light.
So with that I have a one splice into the chassis harness instead of a multiple splice job. Also if you use a converter box you can connect the NEG to the rear chassis ground which is also located behind the spare tire mount.
Hope that helps.
