I believe on that chassis it should be around the front/drivers side , behind the headlamp..
It is a cylindrical can in the refrigerant line near the condenser [ condenser is the coils in front of radiator, behind the aux fans]
It will have the two sensors I mentioned , plus a little glass window on top
[ refrigerant eyesight glass]...if you can not find it , post again... but just look around in front and you will see it.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=21H0MN3YU28W14RKXF&year=1992&make=MB&model=500-E-001&category=R&part=A%2FC+Receiver+Drier