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.

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