MAD electrical upgrade question??

I choose to believe the fault is before that gauge. If not the male/female service disconnect on the fusible link, the link itself, or the bulkhead terminals. I wont argue that OEM amp gauges have failed but those failures are far out numbered by those other members in this current path. I challenge you to prove the instrument itself is the fault.
Any "work around/bypass" method suggested above would work. All lead to the same end result too. A dead instrument. Without some indication you cant know if you fixed anything nor will you know when there's a problem tomorrow.
Maybe you already have or plan to have another instrument mounted somewhere.
Converting the OEM amp gauge to a volts gauge is the most practical route but... more effort than R&R the panel is required. That doesn't address those more typical points of failure either.