The left rear brake light on my 1971 Mach seems to have a mind of its own right now. When I hit the brakes alone it does not light up, when I switch the hazards on it fails to work as well. Only when the right turn signal is being used it lights up. The tail light was working fine a couple weeks ago. I have checked all of the wiring and connections, even replaced the brake light switch to no avail. I am completely stumped. Please help. Thanks.