Ref: Schematic in Post #11.

The mod can be easily done on a hobby table with the necessary materials, parts and tools. The relay array for the heating circuit load isolation can be a simple wiring board with relays mounted and wired as required for the circuit. Made as an assembly the job of wiring it to the vehicle becomes less of an odious chore. I think we're all mentally spring-loaded to the thought that everything we do requires real work and nothing less than lying on our backs with grease, grime and bits of falling rust our only companions.

For the ambitious -

The Heat Mode Selector Switch can be repurposed to energize the load contacts of one relay to place the blower in Low as well as run a series-parallel circuit for contact (load) voltages to an additional three relays for the M1, M2 & Hi fan speeds. The Blower Switch can be repurposed to select relays M1, M2 or Hi by energizing their respective coils. Coil voltages for the relays will be small current draws so an additional wire can be added in isolation from CR43 for the coil voltages of each of the 4 relays. CR43 will provide load contact current for all relays. The fabricated relay control box could be mounted adjacent to the fan speed resistor panel with a couple self-tapping screws. The coil (Heat Mode and Blower switches) voltage protection circuit could be a standard automotive fuse holder fitted externally on the relay box. This configuration retains the normal use of the vehicle HVAC controls but removes the heavy switching loads from the Heat Mode and Blower switches by routing them to relays with suitable contact ratings. The fabricated relay box will have 5 input wires - 1 coming from the Mode Switch to the relay (coil) controlling the fan Low speed, three for each of the the M1, M2 and Hi fan speed relay coils coming from those corresponding positions on the Blower Switch, and CR43 which will be the input load wire across the contacts of all 4 relays. The outputs will consist of 4 wires corresponding to the normal Low, M1, M2 and Hi terminals on the fan speed resistor panel.