Maybe too simple, but have you checked your washer jets? It's a small orifice, so it doesn't take much to clog them. Acoustic guitar string--peferably the high E--is plenty flexible but rigid enough to use a a tool.

(If "not engaging" means the pump isn't running--it's not likely the jets)

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Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)