2000 caviler cd changer installation

Hold on a second before you go cutting into any wires.  2000 and up Cavaliers do not have an accessory wire going to the radio. They have a databus wire, which is wired into other components like the guy at the store said. This databus wire tells the factory radio when to switch on.  You will need the Metra 40GM10, and 40GM20 antenna adaptors.  The constant power can be tapped from the factory deck or any other constand power source. For accessory power in these I usually tap into the BCM switched fuse on the left side of the dash.

You don't need to remove the radio to do a changer.

Get yourself a Pioneer or a Sony, because the power, antenna, and controller cables are all run separately.  Peel back the carpeting on the right side of the trunk, and you'll see that the cable can simply be unplugged from the underside of the antenna.  I'm pretty sure you don't need any antenna adapters back there either, but you'll have a tough time sliding the Sony/Pioneer modulator box directly onto the factory antenna, so buy yourself an ordinary 12" antenna extension---you'll see what I mean when you go to do it.

For constant and switched power, just find it anywhere you can.  Like Chriswallace says, the fuse box is a sure bet.  Also, there's a thick bundle under the left dash leading out to the hood, and thickest wires in that bundle are the main ignition-switch wires--- red is constant, brown is accesssory.

Pioneer changers come with a short ground wire---just ground it in the trunk---plus, I'm pretty sure the changer will also get a ground just by screwing its body to metal.

BCM stands for Body Computer Module or Body Control Module, depending on which GM car you're talking about.  You will probably find ignition power at the BCM if you poke around long enough, but you're not likely to find accessory power there---which is what you want.  "Ignition" means it only gets power while the key is switched to Run, and "accessory" means it gets power when the key is in the Accessory and Run positions.  You want to be able to play CDs with the engine off, right?

Probably the easiest thing to do is just grab the main ignition harness under the driver's dash, open up the tape with a razor, and go for the thick brown wire in there.  Make sure you keep the Pioneer-supplied fuseholder in place----if your wiring should ever short out, you want just the Pioneer fuse to blow---not the car's big fuse that powers the brown wire.

Also, if you can think of any item in your car that only works with the key in Accessory or Run, then you should be able to get power there.  The problem is, I can't think of many things that work that way on a GM car-----cigarette lighter is constant, power windows are in Run only, power mirrors are constant.....you get the idea.