I had the same problem when I first bought an SD. Pull the turn signal relay out from under the kick panel below the steering wheel, pry it open and clean up the contacts and relay. Let us know how it works out..

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'79 300D, Ivory "Gunther" ......going
'81 300SD, Metallic Blue Silver, The Grey Ghost.....going
'87 300D Turbo ...gone
'05 CDI.... gone