The starter solenoid on a Ford Windstar isn't a separate, easily identifiable "switch" like you might find on some older vehicles. It's integrated into the starter motor itself. The starter motor is located on the engine block, usually near the transmission bell housing. It's a fairly large, cylindrical unit with thick wires attached.
You won't be able to easily access or replace just the solenoid; if it fails, you'll typically replace the entire starter motor. To find it, you'll need to look underneath the vehicle, near the engine block, and identify the largest electrical component in that area. It will have at least one large cable going to the battery.
