FWIW here is the same thing installed in my 65 Chevelle. I think Pontiac used a little chrome bezel and mounted it horizontally under the dash, but this is how they put them in Chevelles. The switch and wiring are all the same as Pontiac though.
https://www.youtube.com/watch?v=ZbOWf33oT7k
I *think* this one shows more how Pontiac installed these. This was in my 65 Chevelle wagon. The bezels actually used may have looked different for Pontiac. But like I said above, the wiring and switch are all the same. In this instance I used a reproduction switch and had to rewire it a bit to fit the A-body system, but that was just get the wiring into that little half moon plug connector. I used my original referenced above as a guide along with a wiring schematic I found in a dealership accessories install manual.
https://www.youtube.com/watch?v=hqQenB2mceU