You'll probably find that your Airbag indicator lamp isn't illuminating - I believe it should illuminate for a few seconds when the ignition is switched to the 'Run' position. If there's a fault in the system but the lamp isn't working then the system beeps instead, to alert you to the fault condition.
So the lamp should be replaced, following which the lamp will flash to indicate the diagnostic trouble code (DTC).
This thread in the Ranger forum explains how to interpret the flashing of the lamp, to obtain the DTC.