Unless I am reading the wiring diagram incorrectly, the middle bulkhead connector has:

Ignition switch #1
Ignition switch #2
windshield washer
turn signal switch
ammeter
temp gauge
oil pressure gauge
alternator

I'd leave the bulk head connector disconnected, make up a short jumper, and go through the terminals one at a time until you find the culprit. You are absolutely going to need a wiring diagram unless you are one of those lucky ones and just fall into it. There are two blacks in that connector, one larger than the other. That's the one I'm betting on as there is a splice on the passenger compartment side that feeds a bunch of things.

Just remember the alternator wire and the battery wire are tied together through the ammeter.