Some are automatic and usually turn on when an audio signal is present. They'll turn off a minute or two after the audio signal is no longer present. I have one that works like this but the only gripe I have had so far is that the modulator turns on as soon as I start the van, even when I'm not using the video system. There must be some type of signal or spike in the RCA cables when the van is first started. This in turn interrupts the FM radio antenna signal so that I can't listen to the radio until the modulator shuts itself off a minute or two later. I think I'm going to have to splice in a manual power switch for the modulator to take care of this gripe.
My system uses a Y antenna cable (one male plug goes to the radio, one female plug goes to the van antenna and the other female plug goes to the FM modulator). Therefore, the antenna signal does not pass through the modulator and it does not matter if it is on or off. As far as I can tell this set up does not interfere with the antenna, the volume from the FM modulator is a little bit low so you have to turn it up when playing the audi through the van's radio. It probably has to do with the output signal of the modulator...anyway you can try this set up and see how it works for you!
Thanks for mentioning the Y adapter. I picked one up and installed it so that the Honda antenna didn't have to go throught the modulator. It fixed my little problem with FM radio interference while I waited for the modulator automatically shut off. Thanks again.