1987 BMW 528e Stop Lights

I recently bought this car, naively. I was told that the bulb of one of the stop lights was out. This was after I handed over my money and was dropping the seller off at his house. I know, I am an idiot for not thoroughly inspecting this vehicle.

Turns out, neither rear stop light works. The only working stop light is located in the rear window at the top.

(Ialso found a receipt in the vehicle for 2 new stop lights purchased 2 months prior. There is no package of new bulbs waiting to be installed.)

It's not a fuse. Can you provide me any clue as to what I am dealing with.

Thanks