Rain Sensor

My wife has a 1999 E320 4Matic, she is complaining about the rain sensor not working . I drove the car in the rain and she's right. I even removed the dash cover started the car and drove in this great Oragon rain. It does not turn on, sometimes it works a few times then wont turn on again. Is there a sensitivity adjustment or anything anyone can help me with? Used search to no avail in forum already. Thanks, shes driving me crazy!