I think I gave some poor advice in my previous post about how to isolate the short by removing the bulb sockets. It won't hurt anything if you follow it, but I don't think it will be very productive.
Go to the 1967 Wiring page of the First Gen Site library and examine the 2 diagrams (print both out and piece the halves together). On the diagram, find the orange wire that goes up from the right edge of the fuse block. That is the wire that is supplied by the Stop/Taillamp fuse. If you follow it, it goes to the headlight switch and then to the stoplight switch. The short must be in one of three places: the taillamp circuit, the stoplight/turn signal circuit, or in the fuse panel/dash harness.
To narrow it down, put the test light across the taillamp fuse holder with no fuse installed and verify the test light lights up. Then try unplugging the connector from the stoplight switch (it's the easier one to get to). It's a 2-wire (orange and white) connector going to a switch near the brake pedal arm. If the light goes out, the issue is somewhere in the turn signal switch or turn signal/stop lamp wiring. As an additional test, you should now be able to put a fuse in without it blowing (only while the stoplight switch is unplugged).
If the test light stays on, leave the stoplight switch disconnected for the moment and unplug the connector from the headlight switch. It may be possible to get to it by reaching up from underneath, or you may want to remove the instrument cluster to get better access. If the test light goes out when the headlight switch is unplugged, then the issue is somewhere in the taillamp/license plate lamp wiring. As an additional test, you should be able to put a fuse in without it blowing and you should be able to reconnect the stoplight switch without the fuse blowing.
If the test light is still on with both of those connectors unplugged, then the issue is somewhere in the fuse panel or the dash harness.
Try these tests and report back with what you find out. That will help to narrow down the next steps to find the actual short.
