I drive a newer (2005) Ford f-150 (mine is an extended cab). I read in a service bulletin In these trucks (2004 and up) the air bag gets turned off automaticaly if the passenger does NOT weight ~100lbs.
Have you installed the seat in the truck to see what happens? There is a light on my dash that indicates when the Airbag is disabled.
EDIT: Here is the text from my owners manual:
The front passenger sensing system is designed to meet the regulatory
requirements of Federal Motor Vehicle Safety Standard (FMVSS) 208
and is designed to turn off the front passenger’s frontal airbag if:
• the front passenger seat is unoccupied, or has small/medium objects in
the front seat,
• the system determines that an infant is present in a rear-facing infant
seat that is installed according to the manufacturer’s instructions,
• the system determines that a small child is present in a forward-facing
child restraint that is installed according to the manufacturer’s instructions,
• the system determines that a small child is present in a booster seat,
• a front passenger takes his/her weight off of the seat for a period of time.
• A smaller person, such as a child who has outgrown child restraints,
or a small adult occupies the front passenger seat.