Re: Humbucker acting up?
Between all the different troubleshooting scenarios & everything that comes after....they all begin the same way. The issue could be on any one of a half-dozen islands....and a multi-meter is your boat! It's one of those affordable little tools that you don't realize how often it comes in handy until you own one.
A simple 2 second probe will give you the pickups resistance/output, which will immediately confirm whether or not your pickup is functional. If the Mo Joe is 12k for instance, & your meter reading is pretty much @ 12k...the pickup is good to go. Once that is out of the equation...the next stop in the path is the switch.
Since you said you have experience with guitar wiring, it's not as likely that your issue lies at the pots or input jack...as they're basically in the same general orientation in the majority of guitar setup. (input hot to middle vol pot lug, vol to tone, vol to switch, et cetera). Once again we're left with the switch, as the likely perp.
A super common issue is with the grounds. There's almost always a whole bunch of them, & even a single cold-solder joint or whoopsie will cut off output completely. But typically ground issues either involve certain pickups or switch positions not working at all, or noisy/crackly/fuzzy type issues due to a loose ground. Since you seem to be getting output; implying that the "general" circuit is essentially complete...besides a cold joint hiding somewhere like the bridge claw or pot casing, that leaves that little son-of-a-switch.
Unfortunately it's difficult to give you any type of accurate advice on how to proceed without being familiar with the specific wiring diagram versus what you ideally plan to achieve...& ultimately a visual of your specific wiring cavity in order to determine if the switch wiring is indeed the issue. I'd run-through the basics such as...
Are your switches' common lugs soldered together? Are they supposed to be?
Is your switch orientated correctly? I've often had the switch & schematic backwards.
If your guitar is rear-routed, & will allow you to keep everything strung & playable while you're tinkering with the electronics....it's so much easier to solve switching issues & get instant gratification! Wiring the hot wire & ground of your pickup directly to the output jack will tell you immediately whether or not the pickup is functioning correctly.
If it is, i'd then desolder a suspect switch lead...& tap/probe it to the other lugs & switch positions until I achieve whatever it is I'm trying to accomplish. Regardless of how many times someone has wired a guitar....nobody is above making a simple mistake or overlook error. Just keep patient & logical...and you'll be shredding at full volume in no time! Good luck, bud!