There is a plug under the seat that will disconnect the sensor that tells the system that the seat is occupied.
Edit: The airbag is activated by weight.
Actually none of that information is correct. DO NOT UNDER ANY CIRCUMSTANCES PUT YOUR KID IN YOUR FRONT SEAT IN A 2004-2005 Crew, and most Extendeds.
Only 2006+ trucks got weight sensors, 2004-2005 trucks got a keyed shutoff switch, which only came in extended cab fleet with no rear seating, and regular cab trucks. The sensor under the seat is just a position sensor which changes the force with which the passenger airbag deploys.
You can install an airbag cutoff switch, but only after you go to the NHTSA and get their permission. You also have to notify your insurance company of the change in factory safety equipment.
http://www.safercar.gov/Vehicle+Shoppers/Air+Bags/ON-OFF+Switch+Requests
This says that you can only have an authorized shop do the install, but I am working on a how-to for our trucks as we speak since I have the 2005 extended cab with rear seating, but the rear seats are too small for a forward facing carseat. Rear facing works, by 1/4" with a chicco keyfit 30. My son is currently 9months old and 21lbs, so I have to get it done soon.
Once I get the work done myself, I will take it to a shop and have them inspect and sign off on it for significantly less $$ invested. The pigtail and switch/4x4 module are available from the dealership, and the BCM for the trucks is identical so once the switch is spliced in, it will work correctly.