The defogger vent slots are in there but that's it. Not drilling the dash for it. The 66 must have had one it is drilled just under the ignition switch in the instrument cluster on the metal dash if that makes sense.
I believe the switch mounts under the dash, it was similar to the map lamp on the passenger side below the glove box for the 65 SS. I will look for my service manual to see if I have an image.