2000 Ford Windstar backup lights

Two things you need to make sure you have at the socket(s) where the bulbs plug into. Power and Ground.

Set the parking brake, turn the ignition on (with engine Off), place gear selector in reverse.

Check for +12 volts at the sockets. If you have +12 volts, make sure your ground wire has continuity to ground.
If no +12 volts, check ground continuity anyway, but have an assistant move the gear select from Park to Neutral several times to see if light at least flashes.

If no light flashing, the back-up switch on tranny may be faulty.