How do you know the AC is working in heat mode? If you have water in the evaporator chamber, it will decrease air flow or possibly freeze,, if the AC is working. You need to start under the hood as advised. Check compressor function, check low pressure line for temp drop, check sight glass for bubbles. Work from the front to the back. The drains are located above the tranny near the bellhousing. Rubber flapper hoses. Will probably need the roto-rooter treatment with pipe cleaners or soft wire.
2000 Dodge Grand Caravan SE
No matter what you fix, there will always be something else to fix..
"Warranty" is just another way of postponing the inevitable.