Had the same prob on my '94 Bronco a few years ago. Had to replace the "clock spring" (misnamed, sliding contact) that wraps around the steering column just under the steering wheel. It is a multi-turn flex circuit that allows a "hard" connection between the airbag control ckts and the airbag itself (I believe it also carries the switch closure signals for the speed control). If its resistance gets higher than an ohm or so, the system flags it as a fault.
IIRC, the part cost about $100 from the dealer. You gotta pull the airbag and steering wheel to get to it.
: