To make a long story short, I purchased the car from someone locally which turned out to be a lemon. After fixing all of the issues except for rear cat replacement the heat does not come on. Only cold air blows. In between repairs the car sat for about 6 months. Prior to that I used Radiator stop leak as part of a temporary solution until I can save up enough money to fix the issue. Needless to say is it possible that the stop leak gummed up my heater core? Can it be flushed clean?

 

I just fixed this problem on my 1999 Millenia.
The door or baffle on the heat exchanger box was stuck closed after being closed all summer.
It is accessable behind your sound system box. To get there I started at the back of the console by popping up the inside of the CD box under the arm rest. Then work your way forward by taking screws out and popping up panels. The sound system is held in by 4 large spring clips, 2 on either side. In my case the heater door actuator motor is a black 3x2inch unit which operates 2 doors on the large white heater box. Try a push down on the upper arm while asking for heat (in my case turning the thermostat heat control off 18C to temp above that). I took my black unit off with much difficulty getting at the 3 screws. The worst screw which is behind a metal frame I accessed with a small flat blade screwdriver thru an open threaded hole pretty much in front of the screw.
Good luck and don't worry about the left over screw or 2 as long as nothing rattles.