Multiple units

HI sparkybeanz, you can purchase a dual source switch to select between the two units instead of using relays, though you can do it with 9 SPDT relays to completely isolate the two if you're only running four speakers. Have a look at the "SPK" here: http://peripheralelectronics.com/periph_products/signal_path_devices.htm

You could also do it by running the low level outputs of your CD/MP3 player into an FM modulator and playing it through your factory head unit (no speaker wiring/switching necessary).

the12volt 

   Sparky, I did this on a Mitsubishi a few years back. The customer wanted a CD player added to his system, but he wanted to keep his AM/FM/Cass head unit too. We set it up so that the cassette deck acted as a "Master" and the CD unit a "Slave". They both shared the same 4 speakers.

   The relay package consisted of 8 speaker control relays and a system switching relay.

   On a typical speaker control relay, the #86 contacts are grounded, while the #85 contacts are run to the system switching relay. The NO contact goes to your "Slave" unit, and the NC contact to your "Master". The common contact goes to the speaker terminal.

   The system switching relay is a bumped up remote lead. You run the NO contact to a fused 5A 12V+, and the common to the speaker control relays #85 contacts. The coil contacts go to ground and the remote lead on your "Master" head unit. A diode on these leads is a good idea.

   The relay package is BIG. I had to find a spot to fit 8 relays behind the radio of the Mitsubishi - not very easy. I would suggest you wire up the relay pack ahead of time and label EVERYTHING. give yourself some length on your wires to allow you to move it around and find a good location for mounting. If you can, wire it into a "into car", "into radio", and the "JVC" harness. then it's all plug in and easier to service.

   You'll hear a reasonably loud "click" from the relay pack as all switch simultaneously.

   Dave

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