The Blower Motor Resistor Pack is defective. Replace it. Replace the resistor module for fan motor speeds. It's located under the dash on the passenger side behind the blower. Need to be a contortionist to change it (have to lay on back in floorboard with legs hanging out of the car....lots of fun *grin*) All it takes it a very small 7/32 socket (for back one recommend swivel socket). There are only 4 screws (3 on blower and 1 on resistor) that you need to remove. Part was less than $20 at local AZ. There are wires at a connector on the blower....trust me its easier to just snip them than to try to undo the connector. The new resistor comes with one anyway.
It is either your water pump or your thermostat
The blower on the back heater may need to be replaced on the 2000 Ford Winstar.
Some of these models had problems with the switch itself; You may want to replace it.
you may not have the heater water hoses plugged in
Because your thermostat is probably stuck open
probably the heater core is clogged
Leave it turned off.... doh
Low Coolant? Bad thermostat? Heater core plugged or restricted? Heater core airbound? Heater control cable not working?
It could be the thermostat on the engine. If the engine does not warm up adequately the heater will never blow hot air.
If it does not blow at all then that is a problem, but if it is when you first start it, the Caddy will not blow air on you until it is the selected temp.
Most likely the bad news that you did not want to hear. The heater core is leaking coolant.
heat core. leak. thermostat. no antifreeze.