The brake shift interlock solenoid on a 1999 Ford Windstar is located
on the transmission shift linkage assembly. It's not a single, easily-spotted component, but rather part of the mechanism that prevents the vehicle from shifting out of park unless the brake pedal is depressed. You'll need to look closely at the linkage connecting the brake pedal to the transmission shifter. It's usually near the area where the shifter cable connects to the transmission.
Accessing it will require working under the dashboard, near the steering column. You'll likely need to remove some trim panels to get a clear view. A repair manual specific to your 1999 Ford Windstar will be very helpful in locating and replacing this component, as it shows diagrams and detailed instructions. Working on this system can be tricky, so using a repair manual is highly recommended.
