Coudl very well be a bad ground. For some reason the wiper system is most susectable to a bad ground in the system.
In my motor home and my 300D if some piece of electrical equipment is not connected correctly or in the case of the 300 if I plug a cb radio into the cigar lighter and key the microphone the wipers go off for one cycle.
Took me a while to figure out that I was talking on the radio at the same time that the wipers went off. I thought maybe a bad switch.
It may take you a while to start chasing down ground wires but keep at it and I bet that's what it is...

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique