Air Bag Light

[Philws4]

Ok slightly odd request coming as the car in question is my brothers 1.8T A6 avant (2001) so appologies to the moderators but could do with some info on where the air bag ecu is located as the warning light has come on and not raised a fault code so cant be cleared!
We have tried "creating" a fault by disconecting a plug under the seat but it will only clear that man made fault.So i think we are in for a new ecu but need to know (a) where it is located and (b) roughly how much it may cost.
Any help greatly received.

Phil