That is if you are using someone elses work
If you made the textures in layers I suggest you make the one you want lighted as a seperate layer. That way all you have to do is copy that layer to a black background and you are done. If you used more than one layer they you will have to copy more than one layer but the idea is the same.
I've used this method on my own originals as well.. and the principle is the same for generating a quick basic illumination from your own original textures.
Case in point.. my TARDIS..
This is its Hull texture.. and here's an illumination map using exactly the same settings as I previously used in PSP after I converted it to greyscale...
What I was trying to do here was show him how to generate a quick basic illumination texture using his pre-existing textures. If he has his former work saved in layers already.. awesome. Then he can add in a black layer over his background texture, or whathaveyou and raise it up until windows or whatever start to disappear. But, if he saved his texture as a merged texture and didn't save his originals (as frequently happens).. this'll allow him to generate some illuminations based off his originals.
Which is why I showed him this method.. and I used p81's FCA3 as an example because it's easy to locate, familiar, and a good practice texture to practice with.