My use would be for a Gramin IQue (GPS/PDA) that gives turn by turn instructions. It has a car cradle with a built in speaker but when I’m “Jamin Out” to the empeg there is no way I can hear the instruction from the tiny and sad car cradle speaker. The iQue does put out good, if not great, sounding audio so it would be nice it I could, plug into the iQue headphone jack, and then when a trun instruction comes up the sound would come over the car speakers through the AUX empeg input. The iQue puts out a pre-announcement warning beep so that would help in the cutover timing.
I tried hacking the car cradle to find a “high” or “low” signal but it appears the cradle amp is always on and it get no such signal.
That’s a good suggestion about the sub amp switch. Mine does the same thing. I don’t think noise would be too much of an issue because I could just turn down the input if that happens (so it would be lower than the music) and I don’t get any noise through the car cradle now. However I imagine sub amps have a frequency “awareness” and only turn on at or around the crossover. So ripping apart a sub amp might require some additional hacking.