I’m not sure exactly how you want this to work, but in the other thread you said:
It seems to me that when you say the mirror “works,” you mean that the player can examine themselves in it. And for the player to be able to do that, they have to be in the room. So the rule that checks for light in the vicinity of the player will be good enough for you!
Furthermore, if by “working” you mean that you don’t want to be able to examine the mirror, then that’s already taken care of! You can’t examine anything in dark rooms. If you’ve defined a custom action that applies to the mirror, then all you need to do is add “and requiring light” to the description of your custom action, as in this. (But you should probably make it so that “look in mirror” and “look at mirror” get redirected to doing whatever with the mirror too; start your game, type “actions,” and then type those commands to see what actions they trigger; I’m pretty sure it’s searching and examining the mirror respectively.)
If you really need to check illumination in a room the player isn’t in, look at this example.