I have two 68 Imperials with real rear air conditioning, a LeBaron and Crown, and have seen others. I also have one with the rear defroster blower. On all three cars, under the dash control, is the label "Rear Air".
On the rear air contioned cars, the air conditioning is controled by the front or normal control unit, the rear air switch does nothing but control the rear air blower.
The rear air conditioned cars still only have one compressor, but they have two condensers, which are mounted together in front of the radiator. On the single, or front only air conditioned cars the correct amount of freon is four pounds, but on the rear air conditioned cars the correct amount is six pounds.
On the rear air conditioned cars the spare tire is mounted flat on the right side of the trunk, like the convertible, but all others have the spare tire mounted in the center on the hump over the rear axle. To give more floor room in the trunk for the rear air conditioned cars and on the convertible, there is a cutout on the right trunk liner that the spare tire fits into.
I have never seen a 68 with the rear heat, but this unit is mounted in the same place in the trunk as the rear air conditioning unit, so assume that the spare tire is also mounted flat on the right side of the trunk. The rear heat vents are on the bottom front of the rear seat, one on each side.
For most or normal conditions, the front air and heat was sufficient to keep the occupants of the car comfortable, so think that the rear air and heat units were ordered only for cars that were normally operated in extreme cold or heat conditions.