My heater works fine if temp outside is 15 degrees or higher but blows only cold air when colder outside.

Did you check you antifreeze. Maybe not potent enough

My first question is - are both radiator hoses hot too ? I would want to know if the engine is reaching normal temp or not. The engine coolant needs to be over 100 degrees to make heat for the heater.
You could flush the heater core by using pressure to push water from one heater hose to the other. You could have air pockets in the cooling system which would cause both problems you mentioned.

Sounds like a thermostat and/ or the system is low on coolant.
Its probably not going to reach 200 degrees when the outside air temp is below freezing, but you should get plenty of heat if the system is working correctly.

this may be a goofy question BUT do you mean flushed the radiator not the heater core. Heater core is on the firewall.
Check the hoses going into the heater core to be sure they are both hot after the car has reached running temp. This will let you know if you have a blocked heater core not radiator.
Just wondering?

It Sounds Like You Have Either A Faulty Heater control Valve Or On The High Side A Bad ATC (Automatic Temperature Control). Check The Hoses That Go Into The Heater Core Both Should Be Hot At Normal Operating Temp. If The Outlet side Is cold Or Luke Warm You Have A Bad HCV.

I have a 2000 pontiac grand prix with a digital climate control system. The heat quit working, it doesn't blow out any heat. Now that it is warm outside the air stopped blowing out any air. The digital temperature on the climate control is not correct. When it states that it is below 30 degrees on the ext and it is warm outside the air wont be cold. Once it starts going above 30 degrees the air gets colder. Today the blower stop working all together. Was told it maybe the climate control unit was bad. You can hear the door opening/shutting when you switch the temp from cold to hot. But there was no heat blowing out whatsoever in the winter time. Could you please give me your opinion on whether or not it is the climate control unit or something else?

Check the heater valve is fully open when in the hot position , this valve or tap often get stuck half open. if all is ok there flush the heater radiator as they can silt up.

You may have a heater valve that is on one of your heater core hoses may be bad.

they run off of the same compressor if rear is cold and front is hot the hot and cold function in the heater box is not working possible cable problem ? or a vacum line under dash

You can put a higher temp thermostat and that should help, but it should at least have 170 degree air right now if it was working right. There are two other things you might check for, the diverter door make some sound when you switch from heat to cool and back, I can't tell you for sure if it is a manual, vacumn or electric but that would definitely cause it to blow cold air. There is also a valve which cuts off circulation to the heater core when your not using it. This valve will be in line with the heater hoses going to your heater core. I have seen some that were actually part of the heater core box. I feel pretty sure one of these will be your problem. Also if your car is low on coolant it would not heat properly. Hope this can help you!