Likely a heater core. Does it smell sweet? do you lose any coolant?
You may have plugged drain vents (rare but does happen).
Sweet smell, losing coolant = leak (somewhere). Mouldy fungus smell?
check for drainage issues. Heater cores can be "fun" to change 1977
T-Bird I did years back took 17.5 hours (but it had aftermarket A/C)
Let us know first if it's antifreeze, then some in here with skill
(not you know who) will give you useful tips.
If you're into saving money & won't be driving it past the summer you
could just divert the heater line to bypass and leave the core empty.
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