Before you start swapping out parts and looking at replacements, it might be easiest to check something simple.

On mine, a slight amount of corrosion on the alternator housing caused the regulator to not have a good ground. I attributed it to the low mounting of the alternator on these cars. Cleaned and added a very small dab of conductive grease and nary a problem since.

__________________
Sam

84 300SD 350K+ miles ( Blue Belle )