Yes there is a meriad of hoses at the firewall. But lets make sure it is those hoses. First, when you run the AC system at full heat does the air have a sweet smell? Hopefully not. That means you have a leak in the heater core inside the dash. The leak would drain at the tranny tunnel just as the AC does with the moisture, this is usually not a problem in the SD. But lets eliminate that first.
It could simply be that a hose clamp is loose. I just had this problem. You can tighten the clamps with a flat or phillips screw driver put using a socket driver is best.
I can't remember from my SDL days if you have to remove anything to get to all the clamps. In the W123 this is necessary, the battery has to come out to get to 2 of them.
So tighten ALL the clamps first, cheapest solution. Also check the mono valve for leaks and possible cracks in the housing. also the hoses may have developed a leak. Check them for leaks/cracks.
Then we will go from there.

Dave

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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