Why don't you lift the parcel shelf and find out what's under it?

I kid, I kid.

I just installed a new kenwood headunit and some very cheapo walmart speakers into the original holes.

Most likely, if your current system works correctly (especially the antenna control) then it was wired correctly.

There isn't a lot to screw up behind there compared to a later model vehicle.

As for the parcel shelf, there's a lot of space under the shelf for you to mount an amp and woofer. I wouldn't get too mad about them cutting it up for that. A lot of guys do that since it's a much better option to cutting up the MBTEX side panels.

The parcel shelf can easily be rebuilt or a spare re-carpeted to match your car's color.

Even better, spring for the rear "kinderseats" so you can plop little ones back there.

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-Typos courtesy of my mobile phone.