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Making a car seat belt chime / "bonging" alarm?

Many cars have a chime when you don't put your seatbelt on. After receiving a fine, my mate wants me to give his car a similar function. Any ideas about how I should go about this? Obviously it only needs to work when the ignition is on.?

Does his car have a light that comes on when he's not buckled up? If yes, it's a matter of finding that circuit and tying in a store-bought buzzer from radioshack or frys or whatever.