Look at the electric diagrams.
The single stage airbag has 2 wires. A brown and blue going to it.
The dual stage has 4 wires, grey/black grey/brown to yellow connector and yellow/brown olive/black to black connector.
attached is a photo of a dual stage plug, not from a Z3 (different colors) but you bet the idea.
just check your plug and the one on the airbag you are after.