Here's a post I found on another MB board:
"I did not like the output of my A/C on my 2000 S500. Called the dealer, told him I had heard of a problem. He said any S500 built before 11/99 is eligible. Mine was built on 07/08/99. So made an appointment for next week and they are going to do the job. He said it is a big job. Push the dealer if you are eligible. If you do not get any satisfaction from them contact MBUSA directly. Usually when they get involved, matters get resolved quickly."
The PWR button I mentioned in the last post should be EC. If you have an EC button, then you have the original non-retrofitted system. If you have the ACoff button, then you have the newer system.
Bring it in and make them do it. They only had my car for a day and a half although other people said it took longer.