The brushes mentioned above are 1/2 inch long when new. Yours are likely much shorter due to wear. As mentioned - very easy to check.

It might help to elevate front of car securely. 2 phillips screws on underside of alternator come out and so does the voltage reg. There will be 2 brushes.

Here's what they look like:

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=mercedesshop&baseurl=http://catalog2.mercedesshop.com/&cookieid=1E50U2ODS1E90TK774&year=1987&make=MB&model=300-E-002&category=F&part=Voltage+Regulator