Rob;

In the Delco based Charging System, the Amp Indicator supplies Field initially from Battery, until the Alt starts making power, extinguishes the AMP Ind (no voltage across!), and it takes a little of that output power to supply Field.

I expect the Bosch (with integral VReg) based Chg Sys is similar, but I know in the early ones, with separate Alt and VReg, there were variations (see: https://sw-em.com/123GT_Charging_System_Notes.htm#1800E_ES_Charging_System_Notes )

An effective troubleshooting trick is to divide and conquer...as in separate the Alt from the VReg, and manually supply the Field, thereby taking the VReg out of the picture...if Alt then makes power, VReg is suspect...if Alt does not make power, IT is suspect...

...if you post a wiring diagram of the Bosch based sys (including internals of the Alt (with internal VReg) I'll have a look at it, and try to give some troubleshooting suggestions...

Of course, it should be said to check the easy stuff first...like Fanbelt tension, connection of Field power to Alt, general wiring associated with Alt, Brush wear (yes I read Alt wasn't very old, but again, its worth checking the simple stuff first)...

Good Hunting!