Yep, brake lights are wired thru the signal light switch, with your test light check for power in and out at the brake light switch and go from there. Could very well be a bad signal light switch or harness connection.
Take both wires off the brake switch on the master cylinder. Use a test light to see if either is hot. If so it is probally a problem with the switch, if not check the fuse. Someone will have to hold the brake pedal down for this test..
Or you can take the wire coming from the firewall off and replace with a hot jumper wire. The brake lights should come on when the brakes are applied: If the switch is good.
These test will help you isolate which side of the m/c the problem is on.
If it is a 65, you may have a mechanical switch on the brake pedal. It is mounted to the flat end of the actuator rod to the master cylinder. Check to make sure both wires are connected, if they are, check the switch for power with a meter or trouble light, you should have 12 v on one of the two wires even with the key off. If you have 12 volts, depress the spring on the switch and see if the brake lights work, if the do, you may have to tweak the flat part of the switch that contacts the brake rod. If you don't have 12 volts at the brake switch, your problems are back towards the supply.