Hi - I replaced the harness in my '86 ES335 with one made by Brown Custom Shop (no affiliation, just a satisfied customer). http://bcsguitars.com/?page_id=19
Used the YouTube videos from RS Guitarworks to get a feel for the project. Took everything out of the access cut in the center block (through the bridge pick up cavity) put everything back in the same way. The cut in the center block was large enough to make that part of the job relatively easy. Also, I had Bill Brown add some long leads to the volume pots so I soldered those leads to the pick up wires (charged an extra $2.00 for that option). I didn't have to solder the pick up leads to the pots, had more than enough wire to work with and can now change pick ups (if I ever want to do that) without pulling the harness.
Oh, by the way, I couldn't remove the pots through the F-holes even if I wanted to because Gibson used Les Paul-length volume pots when assembling the original harness - the shafts were too long to get the pots through the holes! The person who did the assembly also put an extra nut on the volume pot shafts to regulate how much of the shaft came out of the guitar top (see below).
Anyway, the harness replacement is a bit time consuming but not impossible or extremely difficult if you are handy and have basic soldering skills - and the results are impressive. Good luck. Ed