There is no relay on the brake lights on a 66. Just the contact switch on the brake pedal itself. Sounds like a blown fuse or a broken wire on the switch. It's located about 3/4 of the way up on the brake pedal. Get a test light and see if you have 12V on one side of the switch, then depress the pedal with the test light on the other side. If you se +12 there it could be the bulbs.