Though, apparently, back in the `50's-60's, the ladies who wired them up all worked through the F holes. I'm not sure I believe that, but I've been told it is true.
Gabriel
To this day, Gibson still works through the f-holes. Though, when I worked there, there was a single woman who did most of the ES-style guitars, because she was so proficient at getting the harness installed.
The harness is initially wired on a jig, so that the spacing of the pots is correct, and because the shield of the wires is also the signal ground, and is soldered to the pot backs.
The entire harness is collapsed along an axis running roughly from the jack to the rhythm pickup volume control, so it's in a long relatively-thin assembly to feed in through the f-hole. If memory serves, the pickups are installed first, the wires brought out to the pots and selector switch, and everything soldered up before the pot harness is installed. Yes, you test the pickups and controls before feeding the mess into the body.
We used a long metal arm, which had a bend to it, as well as a forked end. The arm was used to push the parts into their holes from underneath, generally starting with the jack first. As soon as a part was fed up through its hole, the washer and nut were used to secure it. One installed part helped hold the harness in general position for the next part to be installed.
Naturally, you need a second test to verify all the pots wound up in the correct hole, but I seem to recall that the length of the shield wires were staggered such that once the jack was in the correct place, the only way for the selector switch to reach its hole was to have each pot in the correct hole.
I think that might be the only way to get the harness installed for the first time (can't use the dental floss trick on the initial install), at least when you're dealing with a semi-hollow body style that has a solid block underneath the pickup routes.