I have no heat. on 2000 Dodge Grand Caravan

I replaced the thermostat, and still nothing, coolant is topped off, someone said something about a valve on on the firewall that might be stuck open. At this point my mind is boggled. any help would be much appreciated.

Asked by for the 2000 Dodge Grand Caravan
You have a broke blend door. You have no valve at the firewall. The blend door is in the HVAC box. Not a inexpensive repair.
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caravans are well known for the climate control to go out.
does it switch over to cold air?