2000 windstar is leaking coolant behind motor and smells with ac on...Heater core?

Yes u have a leaking heater core, below fin d the instructions to replace it.
1997-2000 Ford Windstar 3.8
Heater Core
Inspect ' Replace
Drain the cooling system

Heater Housing

The heater housing is usually under the dash and must be removed to gain access to the heater core. Procedures for replacing the heater core vary with the year, make, and model of car. It is therefore necessary to consult the manufacturer's repair manuals for the proper procedure for replacement.
The heater housing is disassembled to get to the heater core.
Heater Core Remove the access panels(s) or the split heater/air conditioning case to gain access to the heater core. Remove the heater coolant hoses. Remove the cable and/or vacuum control lines (if equipped). Remove the heater core securing brackets and/or clamps. Lift the core from the case. Do not use force. Take care not to damage the fins of the heater core when removing. To reinstall the heater core, reverse the removal steps. When the heater core leaks and must be repaired or replaced, it is a very difficult and time-consuming job primarily because of the core's location deep within the firewall of the car. For this reason always leak test a replacement heater core before installation.