Hope all had a good Christmas.

I am almost positive that this subject had been posted before but I am sorry I can’t find the same exact problem I can only find similar issues.

The Patient is a 1993 400 SEL W140

Environment is Chicago, very cold this time of year.

Symptoms.

Heat only works when out side temperature is above freezing 35 or so. If it is 35 or so out side no problem.
Heat will work at colder temperature after driving car for about 2 hours. The blower will just come on, nothing to move, nothing to shake, no bumps in the road.
If temperature is above freezing heat will work as soon as you start the car.
You will feel hot air from the vents when blower not working.
Rear vents always blowing cold air when heat is working.

Fuses all fine, I turned the ignition switch in every possible way to check that. I know the blower works.

Any help will be greatly appreciated.