Having just got a Barchetta I was cleaning the muck out from under the seat and found that there is a wire that goes into the seat from the floor underneath. What is it?

I think this is your Seat belt system. If you have a crash the seat belt will pull you back into the seat, stop you jurking forward into the window.

Yep seatbelt pretentioners, don't disconnect it, because you'll need to reset a light on the dashboard.

Ok thanks for that,
I have not disconnected them. I work on the basis that if its not broke don't fix it.

Yep Alex is right, just looked now, it will set of your Airbag light if you disconnect them. The seat belts and air bags are on the same circuit.,