Answers don't get any easier than carnut and ohiomike provided.

The voltage regulator is a device held on the back of the alternator by 2 Philips screws. Disconnect the battery, remove the 2 screws, extract the device. Most likely you'll find that one or both of the brushes doesn't have a shiny contact area. That means the spring is worn or the brush isn't long enough to make proper contact with whatever that thing is that rotates inside the alternator. At that point you're halfway to replacing it. Some folks have found replacement brushes. Most get a new regulator.

Caveat - maybe your 93 has a newer style alternator with a plastic cover over the back end. I'm not sure how to deal with the regulator on that syle of alternator but it's not too diffierent. I have one in the garage if you need me to check.

Sixto
87 300D