You'll likely need to remove some trim panels to find it. It's usually a small, red, square or rectangular box. Your best bet is to consult your owner's manual or a repair manual specific to your 1997 F-150. Those manuals will have diagrams showing the location.
Without the manual, a visual search behind the lower dash panels on the driver's side is your next best approach. Be careful when removing panels to avoid damaging any wiring or clips.