I've done basically everything Matt has suggested and my heat still sucks. When I removed the heater core it was clogged with all this gasket maker stuff. I wasn't able to clean it with the chemicals I had on hand but i swear I'll never go through another Canadian winter without a proper heating system. This year I'm buying a brand new heater core!
As for the A/C mini condensor. Mine was all clogged up too. Unfortunately I didn't fab up a replacement screen so sometimes it snows inside my car with the heat on.
