The manual shows the same AC system in the 85 300D and I can't see Mercedes changing the evap box in the W123 model in 1985. So with that said, your defroster vacuum element is just above your gas pedal. Remove the driver lower panel and look for a black and pink element. this is the defroster unit.
A mityvac will help you determine if it is the element or not. I fthe lelment checks out OK then it could be the vacuum switch behind the AC control panel.
Let us know

Look here

http://www.peachparts.com/Wikka/W123Evaporator

And go to the picture

"080 300TD Unclip the connector base on the drivers side case"

That is your defroster element.

Dave

__________________
1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car