What Rapid is trying to tell you is this:

There needs to be a GOOD SOLID ground between three things:

the battery neg
the block (alternator)
the body/frame

This is not always obvious, because cables can become corroded inside the insulation and still look good.

Second, the battery POSITIVE MUST be the same voltage potential as the alternator blue (ignition / power feed)

Some things that can cause this path to be low are:

Poor/ loose connections at the ign switch, or deteriorated contacts in the switch itself

Poor loose connections ANYWHERE in the rather roundabout path from the battery positive, through the firewall, through the ammeter, to the ign switch, back through the firewall, etc.

Rapid gave you some good advice, but you can confirm "the problem" without jumpers by doing the following:

With the car at "low cruise" (good fast "boulevard" idle, and check BOTH with battery fully charged, all accessories off, and again, with lights/ heater on

Take a digi meter, and with the veh. running as above, stab one probe right on the batt neg post, and stab the other right into the regulator case.

You want a VERY low reading, just a few tenths of a volt, if it's over 1/2 volt, you have a 'ground' path problem, I.E. poor ground between the block/ frame/ regulator

NEXT stab one probe into the top of the batt pos post, and connect the other right at the regulator blue. ONCE AGAIN, you want a very low reading. Anything approaching 1/2 volt or more indicates a "path" problem from the blue, back through the firewall, ign switch, etc, etc, and back out to the batt positive.

Since you've tried several regulators I DOUBT those are the problem, unless you are buying from some low quality and the same source.

I doubt VERY much that the alternator itself is the problem. IF pulling either field lead drops the charge to nothing, then the alt is OK in this particular case