How do I replace the bulb inside the "brake -0n indicator,located on shelf behind the rear seats of 1989 cad

I have a 1989 Cadillac Broughm that is in excellent condition. The light y. that is mounted to the shelf,just inside the rear window, has a bad bulb. The maintenance manual that I have for this year Cadillac does not cover this unit. Viewing it inside the car shows (1) screw on each side of the housing. Inside the trunk there is small plate that does not appear removable from inside the trunk.