William,
I had the same problems on the '73, asked my local MB Tech about a new switch, instead he told me how to fix it. I followed his advice and the switch lever now stays in place. Since Mercedes seems to stick with the same part(s) for a long time, I think the switch assembly should be about the same (at least I hope they are).
Step 1: Pull the rubber boot from the steering column. This will expose the two phillips head screws you need to remove.
Step 2: Ease the switch around so you can access the locking plate. You wont be able to pull it out very far, but far enough to access. Work the switch for left and right signals, as you do you will see small indentations where the locking mechanism is supposed to catch. After 24+ years of using your signals, you wore the shoulder(s) off.
Now that you see where the problem is, it's easy to fix.
Step 3: While holding the switch, place the lever down for left, that will give access to the right catch.
If you look closely where the screw driver is pointing, you can see the catch notch. Using a small round file, cut the notch a little deeper. I found the small file to be very tedious and used my Dremel with the small reamer bit (which fit the notch perfectly), took about 5 seconds to cut the notch.
Step 4. Repeat Step 3, holding the lever up to access the left catch.
As you work on the notches, occasionally stop and test the catch resistance. As the notch deepens, the resistance increases. Proceed slowly, if you cut too deep, then you end up having to turn the switch off manually after completing the turn, until the new shoulder wears off a little.
After completing the process, blow out the switch with a shot of air to remove any metal particles/debris. Replace the switch in the column and tighten the screws.
Hopefully, you have the same type switch and that this helps.
[Edited by MikeTangas on 01-01-2001 at 03:00 PM]