[goatless]

I believe the the "other" switch is at the key location, mounted on the inside of the tailgate operated directly by the key. I had a '69 Bonneville wagon that should have been the same. It's been a long time since I had the tailgate apart, so my memory is a bit fuzzy.
There is a removable panel on the tailgate that you can open up to gain access to the switch. It's kind of a pain though as the glass has to be up to access the switch, but the gate won't open if the glass is up and the glass won't go up if the gate is open.