The system works off vacuum, this is supplied to an accumulator on the drivers side of the heater box. If vacuum supply is lost it automatically defaults to supply air to the screen only. Please check the vacuum supply pipe, which is located in the engine bay at the brake servo/vacuum pump supply line. There is a T piece in this supply pipe, near the brake servo. It usually becomes disconnected and simply needs re-fitting.
Tony
February 2004
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