Hi Peter,
Just to clarify a bit, many modern BMWs (all?) have a battery positive disconnect system using a squib to sever the battery supply under control of the SRS in the event of a crash. I've never changed one myself but I know they are replaceable as I've reset the codes for the local body shop after they have.
I would think you could disconnect the squib connector and put in a dummy resistor (suggest 1.5-2.0 Ohms) and see if the code will clear. If so a new unit will be required, if not then its hunt the wiring fault. Almost goes without saying but just in case, never continuity test the squib.
Shaun