I found the lead going into the switch and I disconnected it (for the record it is under the floor mat covered by an extra piece of rubber which on this old car had mostly disintegrated).
However, the problem persists. I am wondering if the switch was not broken? I can move it up/down a very small amount perhaps 1 mm. I assumed that it was jammed, however. Perhaps the switch was not connected properly?
Alternately, perhaps it is something else entirely? Anything other than the vacuum lines that I should check?

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1982 300SD turbodiesel
"Rudolf Diesel originally designed the diesel engine to use vegetable oils" so let's all get back to what they're meant to use!
http://www.ybiofuels.org/bio_fuels/history_diesel.html

If the kickdown switch is not the culprit and the Bowden cable is not the culprit, then you are out of options from an adjustment standpoint. There are internal issues with the transmission that prevent the 2-3 and 3-4 shifts.

So on to the vac. lines.
1. Where do I find them? i.e. how do I know which are the vacuum lines and which are not? Which diagram in the "Vacuum tube diagrams" thread is appropriate for a 1982 300SD?
2. What's the best way to test them?
3. If I find one which is leaking, can it be patched, or do I have to replace it?

Hm, here is something I hadn't thought of before. Since I don't have the manuals, I don't know if this car is supposed to have automatic locks. If so, they are not functioning at all, i.e. when I turn the key in the driver's door, nothing happens to the other locks at all (whether locking or unlocking).

__________________
1982 300SD turbodiesel
"Rudolf Diesel originally designed the diesel engine to use vegetable oils" so let's all get back to what they're meant to use!
http://www.ybiofuels.org/bio_fuels/history_diesel.html