Safety Belt Warning Indicator
WARNING: This page is about a different car, the 2005 Ford RV Cutaway, 2005 Ford Econoline, and 2005 Ford E450 Super Duty. However, it is still accessible from the selected car via links, so may be relevant.
The driver safety belt switch is hardwired to the instrument cluster. When the safety belt is unbuckled, the driver safety belt switch grounds the circuit and the instrument cluster illuminates the safety belt warning indicator. When the safety belt is engaged, the ground is removed, causing the lamp to turn off.