Yes this is a common problem. Its usually the heater motor brushes.
They can be bought on ebay for £12.50
http://cgi.ebay.co.uk/Mercedes-Sprinter-Vito-Valeo-Heater-motor-brushes_W0QQitemZ190363120625QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item2c5286b3f1
Baz
January 2010