what's this box?
 

behind the plastic cover in the battery box there is what looks like the control unit, the op relay, a couple of other connectors. and a silver box with what looks like a knob on top with markings for on and off (o and |). just curious what this is for. anyone know?

Is this it?

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1EQ0PTV2S1JL14YSTK&year=1991&make=MB&model=190-E-005&category=P&part=MAS+Control+Unit


Is called a MAS. Several circuits integrated into it. Before 1990, fuel pump relay, KLIMA(a/c) and other stuff had separate relays. They are all now in this gadget and boy does it hurt your wallet when this baby dies!

yes, that's it. boy, judging by the price, i hope i never need one. wonder what the stealership charges.......

what exactly does it control? what's the switch on top for?

mas relay
 

the two basic thing it does are:
it replaces the fuel pump relay, and the a/c klima relay. so it controls the fuel pump and the a/c compressor, i am sure it does more, i have only had issues related to the a/c and fuel pump control.