Buick Century 1989 brake lights

Hi I have a buick century where the brake lights remain on after the car is turned off. This has only been occuring for the last 2 weeks. The only way to turn them off is to disconnect the battery and then when you reconnect the battery next time you nbeed to go out the lights are off. But the catch is that it is only the left and right brake lights that stay on, they brake light in the rear windown works as normal. Any ideas on how to fix this?