I had an FM radio that could be tuned in such a fashion as to pick up a harmonic of one of the local TV station's audio.

Down at the bottom end of the FM scale? That's not a harmonic, that's the the real audio track for VHF channel 6. In my area, that's KVIE, the local PBS station. I used to go over to a friend's house on Saturday nights and watch some of Doctor Who, leave before it was over, and listen to it on the way home.

The reason you can do that is because the VHF broadcast TV channels are actually split in the spectrum, with 2-6 being one group of frequencies and 7-13 being another group. Some radios let you squeak down a bit past the bottom of the FM band and catch the top of the VHF band.