I had that problem earlier this summer on my SL. It started off as once in awhile. It got more and more frequent. I fixed the problem by removing the pushbutton unit, opening it up, and using a low-watt soldering iron to resolder broken joints.

Been working 100% since then.

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Mike Heath
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino