When I had a random wiper issue on my '85 123 wagon, I took apart the multi-function stalk, cleaned up all the contacts with De-Ox-It or similar from radio shack, and that fixed the problem. The end cap can be carefully pried off and then the whole thing will come apart. I removed it from the steering column to do the work, but it probably can be done if left mounted.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC