At least one of the seat belt buckle switch is faulty or you have a short in the wiring.

I had this happen to me and it was a simple fix.
Look underneath the seat where the receiving end of the seat belt buckle fits into. Follow this down under the seat. You'll see and electrical connector in the wiring. Open this connector and using a cotton q tip dry the inside of this connector out. These connectors sometimes get moisture in them from condensation. One this connector is dry and clean snap it back together. The problem should be resolved.