Try spinning the fuse for the wiper. Corrosion can cause a weak connection. Also try removing the combination relay for the wiper and shake it gently. Hear anything loose? Either pop the cover and try resolder bad joints with fine tip soldering iron or replace.

Haasman

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'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)