To be certain, you should:
* Consult your owner's manual: This is the best resource for the specific location on your truck.
* Look closely near the passenger side of the firewall: Shine a flashlight and carefully examine the area. The fitting may be black and fairly inconspicuous.
* Check online forums: Search for "1999 Ford F250 Super Duty AC recharge port location" in online forums dedicated to Ford trucks. Pictures from other owners may help you pinpoint it.
Important Note: Recharging your AC system requires specialized tools and knowledge. Improperly charging your system can damage the compressor and other components. It's strongly recommended that you have a qualified automotive technician perform this service.