i have for a while (as will many others) put up with an airbag warning light after replacing non airbag seats with airbag seats, as the wiring is incompatible the warning light stays lit.
i recently broke a 156 for spares, while doing so i realised that while the seats had no airbags (1999 model, base spec cloth) they also had no pressure sensor in the passenger seat and therefore no wiring for the problematic sensor,
i have today replaced my 1998 2.5v6 with the airbag ecu from the 1999 2.0ts which had no seat sensor as standard and all my airbag warning lights are now behaving as they should with no connection to the front seats
so if you are having the same issue then get yourself down to your local breakers and find an airbag ecu from a car with no pressure sensor in the seat (a quick check under the passenger seat for the wiring will confirm if it has the pressure sensor in the seat or not)
10 min job to swap over (disconnect the battery 1st)
1 wiring plug on the ecu and 3 10mm bolts holding it down (located between the driver and passenger footwell under the dash on the transmission tunnel for those unaware of the location)