[doubleclucker]

Up here in Canada the heavy duty heater option wo/AC consisted of and extra position on blower switch low- med1- med2 -high. there was an extra wire to the resistor and and it was shaped similar to the a/c resistor with the extra circuit for almost no resistance so the blower motor blew faster than normal and it was a little bigger armature with the cooling tube added to it .
It was easy to retrofit because the blower wiring harness was separate but it still plugged in to the main harness at the blower switch (brown) and at the blower box you could drill the hole for the cooling hose and enlarge the hole for the resistor. Easy way to tell HD or not , is the extra position at blower switch or different shape resistor at box.
Parts-
Blower box (or modify)
Blower motor
Blower cooling tube
Cooling tube to box plastic adapter + screw
4 position resistor
4/wire wiring harness to switch
Extra position blower switch