The e320 blower has a regulator that often burns out. Without this functioning, the fan motor will push out little to no air. You can not purchase the original replacement separate from the blower, but you can easily adapt the regulator from the s320 to your blower. Only the wiring needs to be slightly modified. You can purchase the regulator for around $100. I bought mine from the site below about 3 years ago - it is still working great...

http://www.importecwarehouse.com/Mercedes-Blower-Regulator-W140-ALL-S320-S420-S500-p/mb1408218351.htm