It may well be the airbag wire, but not necessarily. On my '03 156, the infamous airbag connectors - now both removed - were housed in a little drawer/tray sort of thing on the bottom of the seat. Attached ti the bottom of this tray is a yellow cable with a white connector which runs into the floor along with the airbag wires. My best guess is that it's wiring for heated seats, which aren't fitted to my car. So, short version is that you should check to see whether there is a box with airbag connectors under there too.