Audio sensing circuit.

I  need help. I cant find this anywhere on the internet. And hopefully someone in this forum can shed some light.

I am not sure if I need some sort of trigger, low to high type convertor or gain control or relay or amplifier.

Heres my dilema. I need some sort of audio sensing circuit. When there is any audio on an RCA pre-amp level line, I need it to trigger +12V. So when an audio source is turned on, this circuit senses it via the rca, then triggers possibly a relay for +12VDC. 

How can I go about making this work. I tried a PAC trigger that said it would trigger off of 0.7V, but the voltage is to low on the RCA.

why cant you just use the remote turn on lead going to your amp? otherwise i would suggest just using a ingition 12vlts or something on that line.

I can't do that. I have a radio that has an AUX input port. But the only way to get into AUX mode is to put constant 12VDC on a certain wire. When the 12V is removed, the radio comes out of AUX mode. So I wanted the radio to go in and out of AUX mode whenever the AUX device is power on/off.

The AUX device could be anything Satellite radio, DVD player, MP3 player, Basically anything with a pre-amp level audio output. So I figured the only way to make this work is to somehow trigger +12V, when their is a signal on the input RCA.

Any more ideas would be greatly appreciated.

I am not sure I follow what you're saying. The way I have it hooked up now, is I have a switch that switchs +12V to the "AUX" line on the radio. This sets the radio into "AUX" mode.

I want it to be automatic, with no manual intervention required....i.e. no need to flip the switch.

So I dont know where the switch comes in play that you're talking about. The voltage sensor sounds good, but wouldn't it always sense the voltage is present feeding the aux unit, whether it is powered on or off, the voltage would still be feeding it.

what turns you aux source on and off?

You good with electronics? if so, take the cover off of the radio tuner and get a DMM. Start where the power goes in and follow it, there should be a relay in line with incoming power that turns the unit on. All you would have to do is sodder a wire to that and then wire to relay to power the aux input. Is it pionner or kenwood? If its pioneer, i'll go take mine apart and tell you exactly where to put the wire.

I knew that was an option, but the thing is, I sell a custom made AUX input cable. So right now I am selling them with a switch to turn the radio into AUX mode. I just wanted to offer my customers an easier way to switch the radio into AUX. automatically when their aux unit is turned on.

Right now I am tinkering with an audio-amp chip and a relay. But so far no luck. The amp puts out 9V whether or not the rca input is hooked up or not. The RCA input is between .2mV and 8mV.

Thanks for all your ideas so far.