If you are going through that much trouble - I wouldn't use the original speaker wire (or fader controls). It's quite cheap in comparison to good speaker wire, and you may (probably) hear the difference.

Run the RCA cables and speaker wire down the drivers side of the car; run power wires down the passenger side. Connect your power wires direct to the battery - and use heavy duty wire. Do it right and you won't have to deal with issues down the road...

I'd recommend reading a book before starting this. The work is not hard, but just use it to make sure you don't do something foolish... I read Car Stereo Cookbook (by Rumreich). Took less than a day and it really helped me get comfortable with the job.

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1995 E300D - 225,000 miles - White Exterior, Grey Interior - original owner