Where is the expansion valve located on a 1999 ford Windstar?

The expansion valve (TXV) on a 1999 Ford Windstar's air conditioning system isn't located in the same easily accessible place as, say, a Schrader valve. It's part of the refrigerant drier which is usually located near the firewall, on the passenger side, within the HVAC system's high-pressure refrigerant lines.

You won't readily see it; it's integrated into the drier. To access it, you'll likely need to remove parts of the HVAC system or at least have a very good look at the drier itself. It's not something you should try to locate and service without proper HVAC experience and equipment. If you're having AC problems, taking it to a qualified mechanic is recommended.