Under the drivers seat of my 280Z is a green box. It was once part of the electrical system but is no longer attached to anything. What was it???

Cheers....

"Fasten seat belts" ? Seat heating ?

Early generation cell phones had power supply often under the seat with the handset dash mounted. If it is that there would be a coax antennal connection to the box.

It's more than likely the seatbelt warning/interlock.