Ford 2005 F250 Owner's Manual page 181

Driving
Make sure the corresponding lights illuminate or illuminate briefly. If a
light fails to illuminate, have the vehicle serviced.
• If the driver's safety belt is fastened, the
light may not illuminate.
181
2005 F250/350/450/550 (f23)
Owners Guide (post-2002-fmt)
USA_English (fus)