Wow, somebody who's had more Lagunas than me!
It's definitely supposed to go off if the switch is in the off position, I'd check that the switch is working properly and check the continuity of the wiring with a multimeter (obviously, disconnect the battery for ~10 minutes before touching anything connected to the airbag computer
Or better still, disarm the system with CLIP if you can)
I haven't had one with the option to disable the passenger airbag yet, I think that came about in 2002/3. Can't see myself needing to put a child seat in the front of my car so it's one less thing to go wrong