The other Romex in the pan is a grey cable.

It is a 2 way switch with a ground screw.

I have not separated the wires on the wire nut yet, but I will try that.

I'm not sure I follow the last paragraph. I have measured the voltage on the bottom screw of the switch to the ground wire in the switchbox, and got ~120V, switch off or on. With the top screw and ground wire, I got 120V when the switch is on, but 0 when off.

Here's where I believe I made a mistake: I tried to measure the voltage between the top and bottom screws, which immediately made a rather large spark and blew the circuit breaker. I did not have anything connected to the fixture.

OTOH, I know which breaker that switch and fixture is on, now... which is unfortunately the same breaker the living room light+fan, exterior lights, utility room light, and outlet the AV equipment is plugged into. So I'll need to wait to find the right time to kill the power.

I'll get you the pic of the switch as soon as I do that, as well as the voltage test on the two connected conductors in the pan.

Thank you very much!
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I am the Doctor, and THIS... is my SPOON!