Gulp, it is indeed a plastic car.

Don't worry about that too much. Since you'll already be running your own wiring, then an additional ground wire to the amp won't be a problem.

I did however have the whole car in pieces to run cables for the autochanger in the boot. Have not run speaker / amp cables before.

That's pretty much the same as running the cables for the autochanger. In fact, you may be able to re-use some of those very cables. If you've already been through that once, you'll do fine with the amp installation.

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Tony Fabris