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When an installer replaces a windshield, they are responsible for the glass and the accompanying rain sensor working. If either doesn't work (I've had a non-OEM windshield that had a defective radio antenna, was replaced with a second that was wavy and nearly made you sick looking thru that are, and finally replaced with something that didn't have those defects. They didn't argue in replaceing them, as they were all a P.O.S.
Don't try to trouble shoot what they have done wrong. Just insist that they replace whatever they need to and make it acceptable. That is your right.