Ok, problem solved. Apparently when the windshield was changed for the PO, someone did a half-assed job, and left out a critical step. When the rain sensor gets attached to the windshield, a special gel must be applied to the back. Mine had no gel. I got advice from the manufacturer of the windshield in my car, and they suggested a Gold Glass Group GGS 901 CP product.

I was referred to a local distributor, ( a vast building filled with windshields and trim), and they supplied me with a hypodermic filled with special gel. Once applied, based on ambient temperature, you calculate a set up time, then stick the sensor to the glass and clip it in. $25 for a single use tube of the stuff wholesale. And I had to practically beg them to sell to me, not being a glass installer.

It rained on the way home....rain sensing wipers worked perfectly.

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2014 BMW 328i Xdrive
2011 BMW 335i M-Sport
2008 BMW X5 4.8i Sport
2000 BMW 528i 5sp