According to the Ford (Helm) Shop Manual "The power supply is a capacitor that will discharge approximately 15 minutes after the battery is disconnected". I choose to be conservative and follow the manufacturers recommendations when dealing with the air bag. I would rather not trivialize something as dangerous as an air bag. This is especially important when making a recommendation to someone who may not have much experience, training, or knowledge about the subject.
Again, the recommended (by Ford) way to deactivate the air bag is to disconnect the battery negative cable (or both if you want), waite at least 15 minutes (per the above), and then disconnect the back up power supply(located above the glove compartment).
Justin,
Verify that you have a code 4 and I'll post the diagnostics for a code 4.