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
SOME CUSTOMERS MAY REPORT THAT ONE OR MORE HVAC CONTROL KNOBS SPIN FREELY, RESULTING IN LOSS OF FAN, TEMPERATURE AND/OR MODE OPERATION.
THE KNOB'S PLASTIC INNER SHAFT MAY CRACK WHEN FORCED AGAINST ITS STOP.
IF ANY ONE OF THE THREE HVAC CONTROL KNOBS EXHIBITS THIS CONDITION, REPLACE ALL THREE KNOBS WITH NEW P/N 16268157.
P/N DESCRIPTION QTY 16268157 KNOB, HVAC CONTROL 3