I haven't read all the responses but checking the brushes is quite simple. Remove the voltage regulator from the back of the alternator (2 screws). If the contact end of each brush is shiny and clean, they are okay, but who knows for how long. If one or both of the brushes has a dark dull coat on it, it's worn too short to make proper contact.

A voltage regulator is around $60 at the dealer and as little as $15 from FastLane. I understand that replacement brushes are available, but I haven't found any information on who sells them or what they cost.

Sixto
91 300SE ... recently 'deregulated' but quickly fixed
81 300SD