Heating problem

[94talonawd]

Ok. So winter is coming up, and the dsm is my daily driver.

I have one problem. The heat doesnt work to good under extreme cold.
When it is anything say above 10 degrees. It will be able to blow good warm air. But when it gets below 0, It's not able to blow any hot air whatsoever.

Ive already replaced the thermostat.

I've checked the heater core to see if it was clogged, I poured water through the hoses and it came back out the other end. So ive eliminated that problem.

I've checked all the cables to see if they were stuck or anything. They all were free moving.

So now im stumped. I'm just wondering if you guys have any other opinions to give before I decide to take it to a shop.