Update:
I found a wiring diagram for the mirror / rear glass defrost circuit. I couldn't make much sense of it with regards to how the switch fits in. So I started there, unplugging and replugging the switch. I then found the solid black wire going to ground near the taillight (as indicated in schematic) and verified that looked solid. I then removed the tailgate panel, found the wires going to the defrost, confirmed resistance, and also found 13.6v with the defrost on. As for the RHS mirror, I felt warmth.
Of course, I fiddled with the switch before doing the checks, so I don't know if I really solved anything. If weather keeps on trend, I'll be able to do a real world check tomorrow morning.
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