I agree. If it isn't working, do it right! Maybe a good set of pots with clean connection will help considerably. Removing the harness isn't to big of a deal. I would remove the whole thing including pickups. If you start pulling components through the f-holes there's always the chance that you could scratch or chip the finish. Fishing the ground wire is always the toughest. I use an old guitar string, feed it into the guitar from the ground wire hole(by tailpiece), connect it to the ground wire & pull it through. You can do the same with the output jack with a low E string with a J bend.

Hey Charlie , how do you like that Epi 56 gold top ??????

PM sent. Didn't wanna hijack.