ammeter/bypass problems

The black is a major main feed FROM the ammeter and goes "generally" to the headlight switch, to the fuse box "hot" buss, to the input terminal of the ignition switch, AND OUT THE BULKHEAD AGAIN to the ALTERNATOR

Disconnect the alternator and try it. Diodes can short and cause this problem

Also to troubleshoot this withOUT endangering the wiring further, disconnect the battery ground and hook the largest 12V lamp you can get your hands on IN SERIES with the battery and ground. Use a stop/ tail lamp, a dome light, or even an old headlamp. If there is a heavy draw or short the lamp will light up bright, but will not burn down anything

THE ONLY 'fuse' IN THAT ENTIRE AREA is the "fuse link" and they are darn poor protection