[Accelo]

There is a black wire that has 2 small fuses in it. What does this connect to? Is it a hot or ground wire?
Interesting enough both sides of the amp gauge are (hot) and fused.
One fuse comes out of the harness and returnees to the same taped section of wiring.
This is on the drivers side and close to the fender to core support attachment point. Likely you need to do nothing there besides confirm the fuse is installed.

The second fuse is also completely sealed and one end terminates at the black connector on the inner fender next to the battery.
This connection must be perfectly clean for the amp gauge to work.

You can test the function of the gauge by turning on the high beams, without starting the motor, and watch for negative needle deflection.
The first time I trouble shot this circuit it took me hours to identity the fuse function and location.
Very few truck have a functional amp gauge because of the shunt system GM used at the time is so dependent on perfect connections.
It isn't an accident they(GM) went to volt gauges in later years.
Cheers.