I went to Radio Shack this weekend to get a relay used in Aydu's solution, and learned that they have discontinued this part (at least in Canada) and no store nearby has any left. Any kind of relay seems scarce there now for whatever reason.

No one here sells any pre-made sensing devices at reasonable prices, either! (About $60 at one stereo install shop, the rest don't even have!)

I have come across a generic automotive relay in a parts store, but the problem is the coil is also 12VDC.

Since the speaker output from the HU is approx. 6VDC, my question is will the voltage be enough to activate the coil in the relay?

 

With the stock 02 radio, you get a constant 6 volts at the speaker leads. This is regardless of the volume setting. I've checked this on my 02 Pro ES and used a simple 5 volt relay to power up and down my sub amp. This is what is being attemped, expept that the Radio Shack $2 relay is not available in Canada for some reason.

The relay is on the Radio Shack web site and is available here in the US (at least it was a week ago when I bought mine).

The stock Pro radio does not have a power antenna lead lead like an after market deck. This forces using a switched lead from the fuse box to power an amp's turn on. As a result, the amp is on whenever the ignition switch is active, which can result in noise from the amp when the stock radio is turned on or off. The simple relay coordinates the power between the amp and the head unit, eliminating this - at least in my case.

I doubt a standard 12 volt relay would work with only 6 volts available at the speaker leads to activiate the relay. I'm far from being an expert on relays, as this was my first use of one. Others with more electrical experience might be able to give some better direction here.

 

The output to a speaker from an amplifier is A/C, thus making the speaker cone move in and out. The voltage will fluctuate across the terminals and is really not a good idea to use it to drive a relay. Although a relay can operate normally with only about 500maH, it could have the potential of damaging the amplifier built into the stereo. Attaching a relay to the speaker output will affect the impedence of the particular speaker and can cause distortion as well as a loss in volume. There are modules out there that can sense when a speaker signal is present and latch on, but you will have to go to a stereo supplier for such items. You can try www.accele.com, but you cannot order directly from them. You will have to get your local stereo shop to place the order for you. I know these things exist, I used to work there.