1994 Buick Regal Brake light issues

My brake lights work only when they want to (when brakes being pressed) Not working at all when I dont have the main headlights on. Sometimes one side (the left) is the only side that works and other times both sides work. We alreadt replaced the bulbs and checked the fuses. Could it possibly be the brake light switch? Please let me know your thoughts if it isn't that then is it possibly the wiring? Thanks for your imput on this matter.

Priscilla in Austin