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
NO HEAT ON FLOOR VENTS
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As
per your problem mentioned. There are few solved help links to
troubleshoot these problems. Go through the list and Click the link below:----
http://whatisbyme.blogspot.in/2012/05/2000-toyota-avalon-no-heat-on-passenger.html
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http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html
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http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html
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http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html
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http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html
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http://whoisbyme.blogspot.in/2012/06/2009-mercury-grand-marquis-ac-blows.html
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http://schematicsdiagram.blogspot.in/2012/04/ac-not-blowing-through-vents-on-2007.html
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http://schematicsdiagram.blogspot.in/2012/04/passenger-side-temperature-is-warm-air.html
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These
will help.
Thanks.
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I'm sorry to hear that your heat only blows out of the floor even when it's in AC mode and the light is on. My parents might have a similar issue at their house. It's also starting to make a high pitched sound. At this point, they'll want to hire and experienced professional who's licensed. https://newtonall.com/hvac
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