Don't give up the ghost yet. I had to extend my own trailer harness on my 88 Xj recently because, these vintage Xj's are 5-pin , which, is common among the boat owner community. I purchased a 5-pin connector and wire loom from a local boat dealer and found it did not do the job. My effort to solder in a 4-pin connector had the same wire/functional assignments as the 5-pin connector from the marina but, the marina connector also had unique spacing's between the pins.
Have you tried a simple Google search for 'Cherokee 7-pin towing adapter?' You may want to begin with a general search for 'Cherokee towing harness adapter' and narrow it down from this phrase.
Ultimately, I found many but, it took me a long time of internet searching before I had success.