On my 87 300E the voltage regulator is part of the replacable alternator brush assembly. About 5-10 minutes work and probably about $35 - $50 for the part. Enter your year model in Fastlane and see what they charge. Fast shipping and decent prices. (For example, on my 300E, the Bosch unit is $52, while the German aftermarket unit is $14: For my 98 C230, the Bosch unit is $40) They often have a pic of the part as well.
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
Click to see 87 300E