I believe the buzzer is part of an under-dash relay setup. There are a couple of ground points under the dash that screw into the firewall and under dash metal framework near the brake pedal pivot area.
While I agree that it may well be a ground issue, I think it is under the dash, not in the rear of the car.
If it were me, I'd put the driver's seat all the way back, and get in there on my back across the door sill with a work light to do a visual inspection of the under-dash area and its ground points. You'll need to remove the black plastic under-dash cover, and probably the lower portion of the dashboard (below the steering wheel and headlamp switch). That will give you plenty of visual line of sight on things. IIRC, the relay is on the left side of the steering wheel column, up under the dash.
Hope this helps.
Cheers,
Gerry