Definately no rust under the battery tray. When I replaced the battery to a 34R optima I pulled the battery tray, cleaned it up (knocked off surface rust and repainted).
I called Park Place Dallas (dealership) - amazing they had the A/C evaporator drain tube in stock (man they carry a lot of inventory for old cars) for the the driver's side. The passenger they could have by noon next day if I needed it (it's only wet on the driver's side).
So, I swung by last night on the way home, $28 my cost. I can't say I'm too impressed with the engineering of this part (I'll post pics later).
But, long story short, I pulled the panel back, found the tube easily and installed it. The old tube had deteriorated to the point where I could see the inner coil spring that is inside the foam of the drain tube (inner spring is there so it doesn't collapse as there is a wiring that crosses over it). I really think this could be designed better. But, then again, it lasted 23 years. If you click the "buy parts" link and look up a/c evaporator tube you'll see what I'm talking about.
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1ZO0K1NU424P0LN0YX&year=1984&make=MB&model=300-SD-002&category=R&part=A%2FC+Evaporator+Drain
Hopefully in a few days I can report back that the car is "dry."
Hatterasguy - I'll post some pics probably this weekend when I get some time.