I have rally gauges with the same issue, that I have yet to investigate.
I have M&H wiring harness modified for an internal regulated alt and new ralley under dash harness and I know the amp gauge works but its not working.
When I talked to M&H on the phone I got a little insight on how it works. There are two heavy gauge lines, one runs to the horn relay and one to the junction behind the battery. To me this sounds like the same point but is suppose to put a small amount of current to the amp gauge. This being the case I should be able to measure battery voltage from each wire to ground.
No one has a schematic of the ralley gauge wiring.
