The stained glass would change the amplitude of some light selectively. But because the FM radio works at different distances, I wonder if it must have some way of adjusting for different amplitudes anyway?
Yes, stained glass is like band filter, they let through a particular frequency range, while reducing those outside the range. Your FM receiver will still lock on the desired frequency as long as their is enough signal strength. It's kind of the same as listening to an emitter that is very far while being very close to another. Of course, it'll stop to work at some point depending on minimum signal-to-noise ratio.
