I had the exact same problem with my '97 S320 and solved it by replacing the pump under the rear seat passenger side. The best price i found was at tis site....www.mercedes-benz-parts-direct.com

I was able to disassemble the unit removed the electric motor which needed brushes. I found the exact motor on the net but the dealer refuse to sell retail, only in quanities of 1000+. I have received and installed the unit, it is a BOSCH unit (i don't have their part number here now but i can get it for you if you like). BTW, i paid $321....

Good luck