The multimeter is more powerful than the very hand of God when it comes to trying to figure out what in the hell is wrong w/ the wiring harness for your baritone guitar. In all other things, the very hand of God is more powerful but in this one isolated case, the multimeter wins out. Here is mine in action (or right before or right after action, I forget):
Basically, all I use it for is a continuity tester to figure out what circuits are being formed when the switches are in various places and for this it works quite well (I also use it to double check that I haven't burned out any pots or switches by overheating them w/ the soldering iron). It turns out the mistakes I made were pretty minor and one mistake was actually the right thing to do in disguise. I temporarily soldered some the pickups on there so I could plug it into an amp and test it. Actually plugging the thing into an amp can be a big help. Sometimes, the multimeter will just pick it up as a break in the circuit but when you have it plugged in, you will hear the sound cutting in an out so you know it's just a cold solder joint or an otherwise weak connection. I ended up fixing some of the grounding and moving some stuff around to avoid short circuits and crossed wires. It is not the prettiest wiring job I've ever done but it works and it should be small enough to fit right in:
I think this is the first time I've ever wired up a guitar w/o getting at least one burn that blisters, which is pretty amazing as absent minded as I can be when I'm frustrated. The only thing that I'm not pleased w/ is that I couldn't get the solder to stick well to the back of the pot I'm grounding everything to so I ended up just grounding all the wires together and then grounding the back of the pot to that. The rest of the stuff will hopefully be grounded by the foil shielding in the control cavity. If not, I've got some more work to do later but I will worry about that when the time comes.