What is part number 1c0959799b?

If this in referance to a VW vehicle, then it is the "comfort / conveniece control unit."

It's the electronics which control things like climate control, central locking, window control etc. It is situated under the carpet, underneth the front passenger seat, both need to be removed to access it. (WARNING seek help as front seat may contain airbags!! - may explode from static electricity etc)

Have you got wet carpets? The drain under the battery can get blocked, leading to rain water getting in through the air vents. Try "googling" passat wet carpets for plenty of blogs!

Hope this is of use

Peter