Regulator gone bad?
I already know I have 12+ volts at the battery (car off or running). I have 12 v on the interior side of the bulkhead connector (car off). I've got a voltage drop somewhere between the bulkhead connector and the gauges. With the car off I get a little less at the battery feed of the fuse box. And even less on the accessory side in the run position. It drops yet another volt there at the fuse box when the car is running.
Ok, let's go step by step. Assuming a fully charged battery (12.7v) and an operating charging system.
You say you have 12+v at the interior side of the bulkhead, so there is no large drop through the bulkhead with no load.
You go on to state that there is a voltage drop between the interior side of the bulkhead, and the fuse box feed, even with no load. So you have the first part of your voltage drop there.
Finally, you say there is a 1 volt drop at the fuse box with the car running.
So, someplace between the bulkhead connector and the fuse box, you have a high resistance connection. Most likely the fuse box connection on the main feed from the bulkhead, but it may also be the bulkhead connector. With no load, a crappy connection can measure close to no drop - yet drop a volt or more under load.
Without knowing much about this car, it is hard to diagnose. Is the alternator output stud wired directly to the battery positive? Is there some sort of ammeter bypass done? KOER, you should have 14v or so at the fuse box.
You definitely have a high resistance connection someplace. That type of problem generates heat. The connection will be warm to hot. It will not get better with time - just hotter.
With the engine running, and the battery charging above 13.5v, check the voltage at the starter relay stud on the firewall, and work your way toward the fuse box. You will find it.
B.