Heater on driver side blows cold air

Does the vehicle have dual zone climate control, where the driver side and passenger side have different control? Sounds like a blend door issue on the driver side. Why did you replace the passenger side, was it blowing hot too when AC was on?

bad driver's side blend door actuator

Look for a loose to broken air hose for the drivers side damper motor.

SOMEHOW I BELIEVE YOUR TWO INTAKE & RETURN HEATER HOSES THAT FEED THE HEATER CORE HAVE BEEN REVERSED. EITHER THAT OR a plugged heater core

Sounds like a blend door actuator failure,Might as well replace both sides

As per your problem mentioned.There are few solved help links to troubleshoot these problems.Go through the list and Click the link below:----
Air conditioner blows air but not cold air http://repairhelpcenter.blogspot.in/2012/07/air-conditioner-blows-air-but-not-cold.html

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Car overheats and Air blows hot at idle? http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html

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Heater blows cold? http://schematicsdiagram.blogspot.in/2012/02/heater-blows-cold.html

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Heat blows cold on drivers side and hot on passenger side? http://schematicsdiagram.blogspot.com/2011/12/heat-blows-cold-on-drivers-side-and-hot.html---------- Hot air on the driver's side vents only? http://repairhelpcenter.blogspot.com/2012/01/hot-air-on-drivers-side-vents-only.html

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2005 Chrysler PT Cruiser: A/C Blows Warm? http://whoisbyme.blogspot.in/2012/06/2005-chrysler-pt-cruiser-ac-blows-warm.html

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1992 Toyota Camry: A/C Control light Flashing and A/C not Blowing Cold? http://technoanswers.blogspot.in/2012/04/1992-toyota-camry-ac-control-light.html

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No heat, blower blows only cold? http://technoanswers.blogspot.in/2012/02/no-heatblower-blows-only-cold.html

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These will help.
Thanks.

Your car is equipped with "Dual Climate Control". Make sure that the temperature control for the passenger side is turned to the "Hot" position. If it is already at the hot position and still blowing cold air, then you most likely have a failed passenger blend-air door motor or a broken passenger blend-air door.

The motor can be replaced with very little labor (a little over 2 hours) but if the door is broken, you are talking about major surgery...total dash removal....about 8 hours of labor.

My 02 Century has dual temp controls . One for driver for passenger. Make sure you don't have the dual climate enabled and just set for heat on your side and AC on the passengers

you cant adjust it ,this requires the dealers computer to re set it,or a phyiscal control has packed up like a vacumn pipe has split where it pushes onto the valve that controls the flap in the heater unit.

I would assume with multiple people having the same problems, there should be a service bulletin on this issue. may want to call dealer with vin # to verify. sounds like a heater control issue under the dash. something i would leave to the experts for repair