its much simple, tell about your level of experience / education in digital electronics. if you will understand it should be as follows using simple switches.
seat belt alarm / light should be activated when some one sits on the seat and the belt is not used.
so you have to have two switches one will turn on (close) when some one sit on the seat. and the other will be on the belt lock mechanism & will be normally on (closed) when the belt is not attached.
connect both in series with the siren / lamp and it will work, there are ways to do it using digital too.