don't worry about the airbags, they're wellard to set off. do disconnect the battery though.
there should be a yellow connector on the seat, possibly more if you have heated seats or anything like that, unplug that but watch you don't knock the fuel cut off switch.
this is located on the front edge of the seat, bolted to the floor - its a rather unassuming rubber topped thing.
undo the 4 bolts that hold the seat to the floor, then lay the seat on its side and remove the plastic surround on the bottom, it should just be screwed on but could be torx screws if you are unlucky. you can then undo the big bolt that holds seatbelt to the seat.
that should set her free.
as said though, dont turn ignition on without the seat connected otherwise it will generate airbag light.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso