The external vents on the cowl are the source of air, so if it's -43C and there's snow/ice on those then your air flow will be reduced. If they're not clear or they melt into recesses near the firewall, that'll also draw moisture in and it'll rapidly re-fog on the glass.

You don't mention coolant temp. Are you seeing around 80 C on the gauge? If not, have you considered a partial block of the radiator with cardboard in order to keep the temps higher? Verified that the aux fan (electric fan in front of the condenser/radiator) isn't jumpered to be always on?

You can also easily remove the blower fan and check that it's not partially blocked.

Here's a link about airflow in 123's. I can't fully vouch for accuracy. Some people have said that they get heat out of the center if they modify the system, but that wouldn't work if it was exactly as depicted: http://www.peachparts.com/shopforum/general-information/295241-scans-airflow-w123-climate-control.html

This link shows the locations of the pods. PeachPartsWiki: Replacing the HVAC Vacuum Actuators

With the exception of the center pod, they're all pretty accessible with removing a trim panel or two. You may be able to trouble shoot the weak airflow with a mity-vac.