I would first remove the fuse for the heater control head for one minute and then put in and retry.
The control head could have locked up on you with a volt spike. The rear defrost switch does not go through the same circuit in the heat as the hvac controls. Could be a broken know also.
Could also be mechanical linkage on side of heater box above accelerator pedal may have broken or popped apart. or if electrical motor control may be actuator.
#76-11-01: HVAC CONTROL KNOB SPINS FREELY (REPLACE) - (Oct 9, 1997)
SUBJECT: HVAC CONTROL KNOB SPINS FREELY (REPLACE ALL THREE KNOBS)
MODELS: 1996-97 CHEVROLET AND GMC M/L AND G VANS 1995-97 CHEVROLET AND GMC C/K TRUCKS
CONDITION:
SOME CUSTOMERS MAY REPORT THAT ONE OR MORE HVAC CONTROL KNOBS SPIN FREELY, RESULTING IN LOSS OF FAN, TEMPERATURE AND/OR MODE OPERATION.
CAUSE:
THE KNOB'S PLASTIC INNER SHAFT MAY CRACK WHEN FORCED AGAINST ITS STOP.
CORRECTION:
IF ANY ONE OF THE THREE HVAC CONTROL KNOBS EXHIBITS THIS CONDITION, REPLACE ALL THREE KNOBS WITH NEW P/N 16268157.
PARTS INFORMATION:
P/N DESCRIPTION QTY 16268157 KNOB, HVAC CONTROL 3
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