Volvo / Common Ground

I'm a little confused as to what the deal is with my Volvo. I took it to best buy to have my CD player put in it. They told me it would take a $300 re-wiring job in order to put it in because the car is common ground. This is where I don't get it. It seems simple to me... Plug in the 2 power wires (Switched/ Constant), the antenna wire, and then run wires to the speakers. Where am I getting lost? Thanks for any insight.

1989 Volvo 740 GL

Well I recently did one of these and the one I did wasn't common ground...it just had a factory amplifier in it  that had to be bypassed, but even that was a piece of cake because I was able to make all of the connections up behind the radio.

Anyways, if it does indeed have a common ground system, basically that means that some (or all) of the speakers use the same negative wire.  All aftermarket systems are not compatible with that type of wiring and would need some type of adapter.  In particular I would recommend the PAC OEM-2.

I was always told by my supervisor that we have to rewire the whole car and that we can't use the adapter which is obviously a bunch of crap; I think they just wanted the extra money, who knows....

If you're at all skilled at electronics even just a tiny bit, with a little patience you could probably do the installation yourself.  Definitely I'd first try to verify that it does indeed have a common ground system because I honestly don't think it does.  Only way to figure that out is to get behind the radio.....

Or you can go back there with the PAC adapter and tell em you want them to put in your cd player and use the adapter and maybe they'll do it.  If you had come to me I woulda said sure I'll use the adapter but if you went to someone who doesn't really know how to use it then they'd just say you have to go all out with new wiring.

That's the thing. There's no radio in there so I just see the plugs (Antenna, Round Pin, Square harness). The harness doesn't have enough wires in it. What I don't understand is why I can't just buy new speakers and new speaker wire and run the wires to the speakers. Why would the entire car have to be re-wired?

How can I tell if it's common ground? The guy at best buy (on my other volvo) just looked behind it and knew right away. Thanks