HI I Have a v8 Discovery which has the same problem. On my car it is becasue the heater motor has just siezed up and won't go round. This can be seen by the fact that the fuse gets warm when the motor is switched on but won't go round. I have yet to take my motor out but in the past with other makes it was fairly easy to strip the motor,(once the damn thing has been removed from behind the dash, which can be hell) clean the bushes and spindle, lightly grease and reassemble. It might be the the carbon brushes (not bushes) are worn or stuck in their holders. Again, just strip the motor and either free off or replace the brushes. If you can't get the proper brushes be creative and modify some to fit. Good luck
Dafyddcoch, November 2007