1965 valiant heater wiring question: what's this wire?

This first picture is looking into the (removed) glove box at a green wire and a brown wire. My question is: what is the (2nd) brown wire ("B"). The green and brown wires ("A") are from the heater control switch. The 2nd green wire ("C") and the 2nd brown wire ("B") seem to feed to the blower motor. The wiring drigram doesn't really show this. C4 (brown) and C5 (green) seem to go to the 'blower motor resistor'.

Why do I ask? Because the 2nd brown wire ("B") is grounded. Not sure if that's right because I don't see that in the schematic.

Ideas?

looking_from_the_removed_glove_box.jpg

valiant_heater_wire_schematic.png