Hi Greg,
Sorry to hear your heater matrix sounds like it may have perforated. It is a bit of a job to get at it but the FAQ is very good. I bought myself some time by making a small U shaped section of 15mm copper pipe and linking the heater out in the engine bay where the flexibles join it on the bulkhead. My head gasket slowly going did for mine, and I had to do that first, so keep an eye on that possibility. Driving through a winter in the SW of England like this gave me all the incentive I needed to fix iit, but I imagine Montreal is a whole lot colder. When I did mine I just slid the seats right back and fought my way to it in the foot wells after taking out the centre console, radio middle bits and side bits. Not very hard but time consuming. Most frustrating moment was prising off the cover of the housing after I'd taken the million screws out and found it had been mastik'd as well! Arghhh!

Good luck

Pierso 1993 940 2.0L turbo manual

definitely a core leak. I had the same issue- I took the core out before finding the FAQ's that starkie mentions- it was a laborious job- and it takes a little time.
Your radiator fluid will leak out into the cabin. it will seep into the sound proofing under the carpet- you will need to take it all out and try to dry it out as best you can- i used a fat-mat soundproofing when I put it all back together and now the heating and A/C work very well-

Here is the link to the section of the FAQ's concerning heater core replacement: https://www.brickboard.com/FAQ/700-900/HeaterCoreReplacement.htm

Here is the link to a post I made after replacing the heater core in a 740 wagon: https://www.brickboard.com/RWD/volvo/1627838/740/760/780/earned_heater_core_merit_badge_replacement.html

Good luck and Happy Thanksgiving!

Randy