With engine running hold both AUTO buttons down on the Climate Control system for over 10 seconds. This should put the AUX fans in HIGH SPEED mode. If not it could be the fans are just worn out. Also, the one on the passenger side is belt driven from the main one on drivers side. If the belt broke, it could be impeding the fan from turning. If you get a replacement online, becareful, the tend to be wired incorrectly. Its no big deal, you just take apart the connector and switch the wires. I had this problem last year, couldnt figure out why the fans were turning backwards.

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Dennis
2006 E320 CDI Black/Black Leather. Nav, HTD Seats HK stereo 32000 miles.
2000 VW Passat 1.8t 300,000 miles as of Dec 09, 2009 original owner
1997 E300d Silver 300,000 miles As of Jan 05, 2010
3rd owner SOLD