Two problems related to frame rust have been reported for the 1999 Ford F-150. The most recently reported issues are listed below.
The frame is completely rusted. The bolts are coming through the bed of the truck. Bumper is rusted away from frame. Unsafe to drive.
Brake line is rusted could cause failure of brakes. Gas tank and supports are rusted replaced once but still all rusted. All frame underneath is rusted impossible to do repairs.