Here are installation instructions for a 1951 Air Flow Heater. The GMC and Chevrolet trucks Air Flow heaters are almost identical; however, the 1954/55 Air Flow heaters are slightly different from the earlier Advance-Design/New Design Air Flow heaters - the defroster controls differ and, as a result, the purpose of the cable control is different.

Maybe someone has a 54/55 accessory installation book?