Rear window defrost for a "66 Baracuda?

There was an option for a rear defroster motor and fan that mounted into a hole that looks like it is for a 6 x 9 speaker on the right rear side and flexible tubing ran to a duct that fastens to the slot you are referring to. I bought a wrecked 65 back in the early 70s that had that option and a factory reverb unit for the radio. I took the body pieces I needed and sent the rest to the salvage yard. Sure wish I had both of those items now.....

Those slots actually defrosted by flow through ventilation too,as previously mentioned. Check out the rubber flaps at the top front of the trunk opening. That's where the air goes out from the slots around the back window.