Oddly, modern Fords (Mk3 Focus and Mk7 Fiesta at least) deploy both airbags regardless. Makes it very difficult to buy a used dashboard from a breakers yard and means higher repair costs for insurers. The timing of the deployment is slightly different if a belt is not being worn though.
Wouldn't take too much notice of my old posts... Seriously though, I have seen a deployed pretensioner flag the airbag light, just not on a 'modern' Ford which is what that particular thread was about. It's easy to see if the pretensioner has fired as the plastic sheath below it goes all wrinkled. In 99% of these cases it is the plugs are fault. But as yours have already been tampered with, all bets are off!
Honestly, I think your best course of action now is to find the airbag ECU, unplug it, and then use a mulitmeter to test the wiring directly back from the ECU plug to work out which is open circuit.