What is the problem? I changed the motor on the direction flap a few months ago. Pretty doable. Complete heater matrix removal means dashboard out and probably both doors as well.
I believe that support brace has to come out, and both front doors have to come off to access the bolts. Some have said that taking the seats out as well makes the overall job easier, though I use the word 'easier' with some hesitation.
This is not a nice job to take on. One historic post recounts two reasonably experienced DIY types doing this on a 500, and it took them a week. Experienced professionals who've done it before can just about exchange a heater box in a day.
There is a known workaround for the all too common snapped spindle issue, but if that didn't work, on an older car, I'd just live without the heater.