If the voltage regulator's been dying for a while, the battery's probably discharged.

Assuming the alternator and voltage regulator are similar to the 80s set-up:

- disconnect the battery
- remove the 2 screws on the back side of the alternator
- extract the voltage regulator
- check the brushes of the voltage regulator
- if a brush is not clean and shiny, it wasn't in good contact with the armature

A voltage regulator is $25-50 depending on brand and source.

I hope that's all it is.

Sixto
95 S420
87 300SDL