Have someone stand behind the van while you press the brake pedal. It works equally well if you switch roles and have the other person do the brake pedal ;)
Check the brake light bulbs, Check the brake light fuse, Check the brake light switch, In that order.
check the brake fluid level. try filling to top. Get it fixed !
Check the fuse, check the bulbs and check the brake light switch.
You are supposed to only have to press the brake pedal. Evidentally your brake lights aren't working. You probably need to see if the brake light switch is bad.
I'm assuming that one of your brake light bulbs is not working
See sources and related links below to lookup Ford Windstar brake light lamp part numbers by vehicle year.
Check bulbs, fuse, brake light sensors.
Check the bulbs, the fuse and the brake light switch.
Check the fuse, check the bulbs and check the brake light switch, in that order.
First check the brake light fuse, then the brake light bulbs and lastly check the brake light switch. If all that checks good you may have a faulty signal switch (AKA the multifunction switch).
If the brake lights have good bulbs in them then your brake light switch has probably gone out. This is located at the brake pedal usually toward the rear of the hinge part at the top of the pedal. If you check this this is the most common reason for brake light failure. There is no one fuse that is dedicated to just the rear lights. There would be several more things not working. I hope this helps some.
The brake fluid level may be low in the reservoir. If the reservoir is low it's telling you the brake pads are worn down. As the brake linings wear down the fluid level drops and eventually turns on the brake light. Have a mechanic check your brakes soon. See "Related Questions" below in case the brake light in question is/are the ones in the taillights. Not uncommon for Windstar brake lights to turn themselves on.