You do not have to remove the heater box. As said above, the cable hooks to the valve on the ds of the heater. There is a tab on the valve that the loop on the end of the cable goes over, then there is a clip that secures the cable sheath, as I recall. Not hard to do.

'64 Sport Fury, 528 Hemi, FiTech EFI, 4-speed, 4.10 Dana 60
'57 Belvedere 2dr sedan, current project in process
'19 Cherokee Trail Hawk Elite
'03 Ram 2500 CTD HO, 6-speed 214,000 miles and still going strong