rain sensor is the separate part itself. It's attached to windshield inside by plastic hooks made with OEM windshied .
There are many problems created by not original windshield and basically rain sensor is not working . Yes it is possible to use old rain sensor IF..plastic hooks still intact and it's relativelly easy to install to windshield.
Other wise you have to change non OEM ( BMW) windshield ( pricey..!!), 2) new rain sensor ( 200 usd!+ tax)
3) PC chip should be regulated by BMW technisian in case to create correlation between this 2 new components
regards