Ive been troubleshooting mine for some time now. think i know what parts need to be replaced but along my research, i came up with the following:
The number of flashes represents a code.
I used this site to figure out what it was
http://www.veryuseful.com/mustang/tech/airBag/
If nothing changed, i would check the mounting of the safing sensors to the vehicle ground, these can corrode. Also may help to disconnect the battery and check the connectors.
Worse yet, did something chew an airbag wire?
Be careful, messing with the airbag wires and sensors can be dangerous.