The diagnostic connector on a '99 is under a round plastic cap close to the hinge for the hood on the passenger-side of the car. But you need fairly exotic diagnostic equipment to read most of the codes from it.

The circuit for the seat belt relays is fairly simple to describe. Fuse #3 in the trunk fuse box is wired to one side of each of the seat belt relays, and the other side of each relay is independently grounded by the roll bar controller.

Diagnosing the problem will require a basic multimeter and removal of the bottom seat cushions (easy). Post back if you wish to proceed.