I'm sure there's probably a thread on how to do this already but I can't be assed to search for it.
The light for the airbag in the seat is constantly lit in my other half's '00 Xantia Activa. I saw a solution in the Lexus forums, but Japan and France are miles apart (Soarer Active aside).
Or it could be the metal safety clip on the back of the side airbag has tripped because the connector plug is slightly lose on the back of the airbag so it puts the warning light on.
Without a fault code scanned from the SRS ECU you aint gonna know what it is.
Disconnect battery, wait 30 mins, disconnect yellow plugs under seats and squirt with switch cleaner, reassemble. I find this works but has to be repeated from time to time.
With seat mounted side airbags the metal safety clip on the back of the connector plug on the airbag inside the seat is always worth a look / press back in