Help-No Heat From Vents!!

I've got a 1997 528i with 285,000 KMS...the radiator, water pump and thermostat have all been replaced (within the last 2 years). The temperature outside has dipped to -15 C and I am now experiencing a problem with the inside cabin vents not blowing hot air through the vents. The car warms ups and the temp needle pins to 12 o`clock without any problems. The vents blow cold air at idle (with temp needle at 12 o`clock) and them warm air when driving the vehicle. I have checked coolant level-it`s o.k, the fan speed setting ( hi and low) work fine, and the wheel located at the middle of the dash vents can be adjusted fine ( red and blue)...I have tried turning off and on the unit, adjusted temp setting up and down and tried to turn the wheel slowly (blue to red) but still not giving off heat as it should.

Can anyone help with some things I might do to troubleshoot the problem and offer any advice on what the problem might be.

Thanks in advance for your assistance.

Im thinking one of two things.

Either the blender door is stuck, not functioning, or not getting a signal/vacuum.

OR
Heater core is plugged by corrosion, or ice.

Ensure that the blender door is moving when you cycle from cold to hot, need to find some info as to where exactly the actuator is(what moves the blend door.)

I look at blend door first.