Triple check all the connections under the seat then take it to a shop that can pull the airbag code. Try an independent auto body shop. A few established mechanics in my area did not have the equipment to deal with airbag system codes so I personally would go to an auto body shop.
Once you have the code you can begin to troubleshoot.
On a side note - I had this happen to me more than once, both times were due to moisture/liquid on the seat sensor. The second time the light came on (seat was slightly damp) I punched the pressure sensor in the seat bottom (where your butt would go) a few times and to my absolute amazement the light went off and stayed off. Make sure to curse the sensor while you are punching it. It was the first time in my life the God of frustration took pity on me. I suspect what happened is that the repeated force did something to the pressure sensor, maybe freed part of it up? No idea.
Sounds crazy but that's just what worked for me.