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81 Stereo speaker wiring advice


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Hi everyone.

I'm in the process of installing one of those Custom Autosound 740 stereos to replace the old cassette stereo.  I figured it would be pretty simple unplug harness, plug in new connections, done... and of course its not.

The wiring behind the radio is hilarious, with multiple pigtails and dead ends and wires jumping multiple terminals in the harnesses.  It took me a full day to just figure out what was going on.  An entire set of wires leading away from one of terminals turned out to all be cut and dangling loose behind the glove box.  Lord knows what was done here.

In any event, after untangling the mess and removing the dead ends, it seems that I have 7 useful wires leaving the area, and only 3 reappear in the trunk, where one of them is split, so I'm guessing I have stereo up front and the rear speakers are wired in series/mono.  My question is does anyone have a sense of where the speaker wires travel and how hard it might be run a fourth wire to get true stereo in the back?

Thanks for any thoughts.  another simple project becoming a rabbit hole, alas.

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Personally, if I were installing a first class sound system in an Avanti and I have. I'd run new wires to the speakers both front and rear and enjoy the sound you paid for. The wiring to the rear runs down the drivers side, along the rockers or center console and into the trunk behind the rear seat on the same side. You'll find there are a couple of clips holding the OEM wires so I'd just drill a new access hole for the new wires.

Also run a good power wire and ground to the radio for good known sources.

The rear seat is pretty easy to get out as well as the rear fire wall as are the trim pieces along the door sill to run the new wires under the carpet. Just re-caulk the firewall and listen to tunes with the new system.

The age of these cars make them a mess wiring wise so I generally just remove what I can and run new wires in neat harnesses.

Edited by Avanti83
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I agree with what Avanti83 said above; it's probably best to rip out the mess you have and neatly run new wires. But, if you want to figure out what you have now, a 9V battery is useful in sorting out speaker wires. Just touch the two speaker wires in question to the terminals of a 9V battery, and you'll hear a noise from whatever speaker the wires go to. 

As for the three speaker wires going to the back, many older stereos didn't run a separate pair of wires for each speaker. They use an independent + for each speak and a shared - (negative). So you'd have a L+, a R+, and one - that runs to both speakers.  

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  • 3 weeks later...


While yes, I agree that running new wire would be great, thats just not a near term project.  What I discovered is that the rear three-wire system is a fairly common thing called "Common Ground" wiring, which is still stereo.  A new head unit or amp can use common ground wiring by first running the four wires into whats called a "Floating Ground Adapter", which then spits out the correct three wires.  Its a $10 little doohickey that fits fine behind the radio and is available from Crutchfield or other vendors.  Took just minutes to wire up and works like a charm.  Heres the link to the unit itself if anyone else needs this:



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