actually, i think I'm havin a flash back of how I got that one out. . . First of all, disconnect the battery, then you have to go in round the back (no pun intended) right in the passenger foot well. You'll have to remove bits of interior trim and the glove box, eventually you'll see the air bag unit and there'll be four screws holding it in on a bracket.
If i remember correctly it was a right *** of a job and a bit fiddly. Hope this helps!