OK, here's another 'not sure but' answer.

I'm not sure if all 68 GMs were this way, but...
On my 68 Firebird, the factory rear defroster was just a fan mounted under the rear package shelf directed up at the window. It made noise, and blew air around, but wasn't really much good as a defroster... At any rate, the switch for it was underneath the lower left corner of the dash, just to the right of where the parking brake release knob is. The switch was tucked up behind the dash, and the knob poked out through a small (I seem to remember rectangular?) hole at the bottom edge of the dash. It had a flat chrome plastic knob with a knurled edge. You rotated it to turn it on and off. The orientation was such that the flat of the knob was nearly parallel with the floor, so all you saw was a bit of the knurled edge. I actually owned the car for a while before I noticed the switch there.
I just did a quick google search. Here's a picture from
a Camaro site...it looks very similar to what I remember, so I'm betting the Nova used the same thing.
UPDATE: found a Camaro picture with the switch...look just under the 'F' in the 'property of' label: