Cadillac Repair: 91 deville heater and defroster, vacuum line, defroster

Question
heater and defroster get warm only on one side of the car driverside blows out cold air

Answer
What mechanical work or ANY kind of work has been performed on the vehicle recently ?. As most problems that occur after a recent repair was made can be traced back to the area of where the repair was made as a vacuum line may have been knocked loose or a wire connection came loose, Look around in the area where work was performed..

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