Along with Whunters suggestion, you might try making sure the lead from the battery to the soleniod is on tight. I found it to be loose on my car and when I tightened it some of my electrical problems went away.

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Sedecrem

82 300SD 290k(needs engine swap)
79 240D 354k with new heart transplant w/220k
82 240D w/auto tranny soon to be manual 4 speed