Hi All,
After a lot of trial and error, I was able to replace my evaporator drain in my '85 w123. I just wanted to share the technique I used. I was not wild about sourcing/using the original tubing, though that might be the better option. In my case, the materials cost about 5$ total, more if you add pipe insulation

DISCLAIMER: This technique is slightly dangerous, make sure you use common sense and don't blame me if you have problems.

Materials:
1" diameter heat shrink (shrinks to 1/2") - 2 pieces about 6" long. I bought this in a 3 foot length, so have a lot left over.
1/2" ID pex tubing (anything roughly that size should work fine) - 1 piece about 3" long.

Tools:
Something to cut the tubing/heat shrink
A small torch with a heater attachment (mine's a bit larger than a pen, found at many hardware stores)
Spray bottle of water
Fire extinguisher - better safe than sorry

Technique:
0) This assumes you have access to the drain - in my case I had the dash out. You want to make sure there's no carpet/insulation/etc near the drain location.
1) Slip one layer of heat shrink over the tubing, so that they are flush on one end.
2) Heat the heat shrink until it conforms tightly to the tubing, and slightly shrink the rest (you want to be able to slip another piece around it)
3) Slip the second piece of heat shrink over the first, but rotate it so the flat spots are 90 degrees offset. Shrink just the area over the tube.
4) Insert this completed piece into the drain hole, and slip the heat shrink over the drain tube.
5) Spray the area under around the drain with some water
6) Use the torch to shrink the heat shrink tubing onto the drain. You'll probably want to access this from both sides.
7) Optional: Add a zip tie or two over the drain to secure it in place. Probably not necessary
8) Optional #2: Add some pipe insulation over the drain to help prevent engine/transmission noise/fumes from entering the car.
DISCLAIMER again: This technique is slightly dangerous, make sure you use common sense and don't blame me if you have problems.

This is a great idea - I've tried the Dr. Pepper bottle method but I don't think I have it quite right and will re-do it your way but with a heat gun which should be less risky than using the mini-torch up under there.

excuse the possibly vague or ignorant question: but what is the minimal amount of interior or dash I need to remove in order to get any kind of access to this tube? is there a good thread - one that shows any pictures? I've seen some scary DIY threads around fixing AC and the amount of dash and center console stuff removed was... insane.

I know from old rust in driver's floor that I need to replace this thing (plus the similar foam temp tube behind the glovebox area in dash has turned to mush). But I don't have any idea how to access the evaporator tube.

There is a guide on the wiki for evaporator replacement that goes into much more detail than I can. Basically your goal is to remove the carpet on the center tunnel, which should mean getting the console and kick panels out of the way. Since i am replacing my dash, i went much further than you should need to.

Good luck

I posted my repair w/ photos a few years ago. I used a dishwasher drain fitting from Home Depot, as have others (PN in post).

I removed just the driver's and passenger side under-dash panels. By now, you are all familiar with that regular chore. Eventually, we will all end up just keeping those panels on the shelf. You have about as much room from both sides, which means "one hand only". You can peek in OK with a flashlight, just not while your hand is in there. You remove both sides because you will likely end up calling a helper for a 3rd hand. The old foam tube comes off easy, because it has disintegrated. The new drain must be folded in, pushing the lip around the plastic stub w/ a long thin screwdriver. You only need the helper at the end (30 min of cussing later), to hold the screw clamp in position as you tighten the screw w/ a 1/4" socket w/ long extensions. You could skip the screw clamp since that adapter is probably going nowhere.

Note that M-B decided to just drip the condensate down onto the transmission. Don't be alarmed when you see a puddle below your tranny. If it ain't red, it is just AC sweat. This only applies to W123 300D cars. I understand the 300SD have long foam tubes run along the tranny tunnel where they work great as "soaker hoses" to wet the whole carpet. Clever engineering.

__________________
1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

I heard about the dishwasher drain fix, and tried twice to make it work - but found that I needed to cut it to make it fit, and got sick of trying to force it in place, due to the thickness of the rubber. Either way works, I am sure, but this was much easier for me.