My 1988 conv. third brake light came on by itself and will not go off without unplugging the bulb. The other brake lights function properly. Any help in solving this will be appreciated. Thanks Jim.

Welcome into the forums there, Jim!

Without seein' a wiring diagram, I'm guessin' the problemm is at the brake pedal switch. There might be multiple outputs at the switch - one for cruise control disconnect, one for brake lights, maybe another for the 3rd light.

Check there first. That's the only 'switch' in the brakes circuit that I know of...