Robin,
did your car come with a rear window defogger / blower assembly from the factory? YOu can tell because the hole will be cut out in the rear shelf. It's possible the rear package tray has been replaced if the blower is not there, so you need to check from inside the trunk. Also I believe it is a large diameter purple wire that runs to the blower assembly, from my memory, I had a 68 with this option years ago. The switch was located below the dash instrument carrier, but looked like a round knob, like you find on the wipers or headlight switches. It's possible that switch failed and was replaced with a standard style switch over the years. The correct switch to my knowledge is not available, nor is it being reporduced by anyone.
Of course it could just be a switch like the others suggested was installed for any number of owner installed options over the years.
Dave