I had the same problem on my 95S420 until I re-read the owners manual. Have to have the headlight switch one click clockwise from the verticle running position. Then it comes on with the regular window washer spray.

Do your dash lights come on when the light switch is in the 12 oclock position? Mine doesn't turn on until the switch is placed one click clockwise and was wondering if I need to replace the switch?

Tower