tHE HEAT IN MY 200 ONLY BLOWS HOT ON THE DRIVER'S SIDE OF THE CABIN....THE PASSENGER'S SIDE BLOWS COOL AIR. WHAT IS CAUSING THIS?

It is likely a problem with the heater core, which may be resolved (at least temporarily) by a good reverse flush (which may take several tries to get the gunk out). This is NOT an issue with the blend or actuator doors and is a common issue with these cars. If you wind up replacing the core itself, the costs will be quite high (likely in excess of $1,000), and may also involve the radiator. Replacement involves removing the dash, as well as disconnecting the A/C (including, of course, evacuation).

First check the coolant level when the engine is cold. Top it off if needed. You could also have an issue with the blend door actuators that control air flow behind the dash. Alex