The schematic on the info site at https://www.alfamontreal.info/Montreal/ColourSchemaAC.pdf incorrectly documents the wiring around the blower switch and indicator light. It shows the indicator light being driven by a red/black wire from the A/C temperature control #74 (the one on the left of the HVAC panel on the dash). In fact, the indicator is driven by a yellow/black wire coming from the 4-position blower switch #72 (the one on the right side of the HVAC panel). This makes more sense since the red/black wire really only turns the compressor on and off. The voltage on the yellow black wire exists at four levels, 0 VDC plus three higher up to about 9 VDC, depending on the position of the blower switch. This gives the blower indicator light the interesting property of not only indicating when the blower is on, but also potentially what level of blower speed is in use, just in case you need to know that.