This is a common problem. Same thing happened to me on the S320 - there is a broken gear in the wiper transmission assy and You can't buy the gear separately. Your dealer will want to sell you the whole wiper transmission assembly, and you'll flip when you hear the price (more than $1000).
Phil at Partshop/Fastlane found me a used one for $300, that I installed myself. Instructions on Alldatadiy.com helped me do it step by step - it was about a 2 hour project.