[jonzcustomshop]

some stuff I found inside the cab:
"no heater" will come with a block out plate on the dash, 60-63 are the same, and 64-66 are the same, chev and gmc.

The 60-63 recirculating heater uses 2 switches (1 fan,1 air), this is mounted in a 4 switch deluxe plate, however the two
outer slots are not cut through in the plastic backing.

the deluxe heater uses 4 controls on 60-63, and 3 on 64-66.
60-63 had a separate fan lever/switch, and 64-6 integrated the fan control and switch with the air lever.

both my 64 chev, and gmc have a thrift-air heater, they use 1 knob that pulls out to control the air vent, and spins to turn the fan on.these heaters have a manual lever that you can switch under the dash (radio) to turn the defrost on and off.
the thrift-air knob is mounted on a 64-66 heater block out plate.

deluxe heater 60-66,and 64-66 thrift -air:
under the dash is an air tube (square) which houses the electrical switch and air control flap, and an air box, which houses the defrost flap. these are 2 separate pieces, and will interchange.

the air tube has a switch/electrical connection on the back side.
60-63 it is a snap in piece, 64-66 it bolts on. the electrical connector looks to be the same all years.
the 64-66 air tube has a square hole cut in the front right in front of the air flap, the 60-63 does not.

the air box 60-63: the defrost hoses are 2 inch od.,and 64-66 (all) are 2 1/2 inch od.
60-63 has a integral metal air outlet, 64-66 has a plastic bolt on air outlet.
64-66 thrift-air box has a manual defrost flap lever