This sounds like a bad voltage regulator, which is part of the
alternator on these cars.

My original equipment alternator on my '82 300 TDT lasted for 250,000
miles.  I replaced it with a NAPA alternator, and I went through three
of them, each lasting about 20,000 or less.  I have a Bosch
remanufactured in there now and it has lasted for 80,000 so far.  The
last free replacement NAPA alternator is on a shelf in my shop.  I am
not sure why I am saving it because I don't really consider it worth
the time to install it.  I got tired of getting stranded with a dead
battery out in the desert (I live near Death Valley).

Get a Bosch alternator.

http://catalog.eautopartscatalog.com/eyc/wizard.jsp?partner=eyc&clientid=boozak&
baseurl=http://www.boozak.com/&cookieid=1ZF0DIWGC2040A7193&year=1982&make=MB&mod
el=300-TDT-001&category=F&part=Alternator

The "brains" of an alternator is the voltage regulator which
is an easy to fix add on thingy on the back of the alternator.

It's a cheap enough part to replace ($18 or so) but can be rebuil;t
with $2 worth of brushes you get from any alternator/starter place.

Here's how:
    http://articles.mbz.org/electric/reg-repair/

That symptom is because of huge draw of power by your glow plug system. It
means your alternator is failing. So I'd put in a new regulator on it to see
if you can still keep your alternator. Two screws on back of the
alternator... very easy job.

Alternator must provide at least 13.5V at idle to charge battery...
Otherwise, it will just drain the battery further.

How old is your battery? Maybe you should have the charging system checked
by Autozone... free of charge.