[Killer Texas GT]

That does signify a heated rear window, does your rear window have the heat strips on it usually reddish orange in color with a terminal on each side. there is also a relay mounted on the fuse box for the rear heater.
The center switch on the light group is fixed on the US imports but does have a function on the European GT-J models, it turns on the head lights, where the US or A-L models has a micro-switch that turns the headlight on when opened.

 

[Killer Texas GT]

switch at the bottom is for driving lights, US option, very few took it. As for the High beam on the turn signal arm,69 and early 70 where the same, button on the end for high beam, later models the 'touch'-back on the arm activated the high beam. The 3 position switch for the GT-J you will have to get from someone in Europe or European E-bay.
The interior light has a three position switch on it, one for when the doors open the light will come on, Center no light at all, the other is to turn the light on at that instant, if it does not work check your bulb, or switch contacts, another someone have unplugged the wires on right side in the rear of the car along the inner fender.