Clicked on your project and it looks like it had one from the factory that's been removed. only heater installed cruisers had the vent on the passenger's cowl. All the hole you need to install a factory were there at one time.
My 66 did not have the rear heater but did have the extra switch hole in the bracket next to the OEM front fan switch, the heater hose has a brass Y at the fire wall and brass plugs for a rear heater hoses inside but the rear floor was drilled for rear heater.
I cant remember where on IH8MUD I saw rear heater was a factory option in early cruisers.
:whoops: I need to stop looking at Mud on my phone. Screen is just too small.
Rear heater was a option in 68. The only thing special is two fixed nuts in the floor behind the passenger's seat. Rear heater is easy to add to your 66. You need to find a second switch with the H in the center of the knob. Where the brass water connectors under the dash are way easier to find then yours from a 66. I had a hard time finding a set of single heater fittings for my FJ45LP-B which came from the factory as a non-heater 65. The water lines running to the back were just rubber hoses in the early years. Didn't go copper lines until sometime in the seventies.The correct location for the rear heater when they first came out was offset to the passenger's side. Not sure how they routed the hoses. I added the rear heater in my 68 so not sure how the lines should have been routed. I just routed them just inside the gas tank. Really no other way to do it. I got the heater out of a early seventies FJ40 in a junk yard many years ago. Stripped the Tees and wiring as well. The wiring was simple for me. I just pluged the switch into the factory harness. Your 66 wouldn't have this. I do remember that the power came off the pigtail feeding the front heater switch.