Alldata has pretty good wiring diagrams, they break things down into sub systems. Worth the money if your doing a large project or are constantly troubleshooting early XJ wiring.
The FSM diagrams, Alldata and a harness out of a junkyard XJ, all together, are about as good as you are going to get, for use as reference material.
I lay my junkyard harness out on the shop floor, print out the pertanant sub systems and have the FSM open to the appropriate page. And then start troubleshooting the harness in the vehicle (for the hard to troubleshoot problems). It sure makes things faster and easier for me. Prior, planning, prevents, pxss, poor, performance, the six "P's".
I got a rather large harness from a decades old computer server setup. A three inch thick bundle of 12 gauge (I think, actually 1.5 mm core area/cross section) wires about fifteen feet long, with just about every color and stripe combination imaginable. I found it at a metal recycling yard. It was in a pile of cable as big as a house.
It's hard to not keep using the same colors, when you make your own harness and gets real confusing later. Numbered tape wheels, take a lot of time to use for marking and the tape usually falls off eventually.