Im new here, and found this forum whilst searching for a solution to the airbag light.
I followed the excellent guide on this forum here (How to fix your airbag light..)
As I already had a standard VAG OBD2 reader, once I had read up a little on the various adaptor cables needed, I just opened it and fitted a switch between the original pin (7 I think) and 3 so I could switch between ECU mode and Airbag mode.
Mine did give an error, said the drivers side airbag had low voltage or was grounded (sorry, cant remember the exact error). The error came back immediately when I cleared it. What I found odd was, when I went through the various items in the "parameters" secton of MultiEcuTool, it insisted that there was someone sat on the passenger seat - there was nothing on it at all!
Anyway, after reading your post just now, I don't seem to see mention of another identical plug and socket under the drivers seat - just the passenger seat. This may be due to the fact that your car is a JTS not a JTD as I have, and as Im new here as well as to Alfas I don't want to come across as a know it all ! may be worth a quick look though, I cleaned both of them, although neither seemed particularly dirty - the one under the drivers seat was a very poor fit though, so I removed the pin headers from the sockets and squished them a bit. Seems to have worked - no more light and no more invisible passenger. Hope you sort yours out.