Dont know about the fiesta but on many cars the most common cause is the wiring under the seat being disturbed, either by the seat being slid back and forward or someone shoving a vacuum cleaner hose in there!
The wiring is part of the seatbelt pre-tensioner circuit, which also "talks" to the airbag system.
Make sure all connectors are firmly plugged in
gsday, April 2009
