I had some issues with trying to get my new Sony head unit to work with my original speaker wiring. It turns out that the original speaker wiring consisted of 3 wires. A common ground, a left and a right. For some reason even with my West Virginian wiring skills to make this work I was overloading the radio and ignition circuits. Radio was red hot and my ignition switch actually MELTED.
So, after a tip from my buddy at Kyle Automotive Specialties, I re-ran the speaker wires from front to back, replacing the existing 3 wire setup. Here is how I did it.
Unscrew the small screws above the sliding door. About 6 including the heater vent screws. This allows you to access the single tube covered wiring harness containing the 3 wires from the front. Using my WV engineering skills, I cut the old speaker wire harness behind the glove box, then black taped two new speaker wires to the old 3 wire harness. Make sure you have enough speaker cable to reach all the way back to the compartment over the rear seat. Also, I had to bend back a couple wire holders above the sliding door.
After that I was able to pull the new wires through the existing channel all the way back leaving enough in front to reach the radio. I then hooked up the rear speakers and the front radio connections. No more load issues on my audio circuits.
I threw in a Bob Marley mp3 CD to break in the new system and now, happy days are here to stay, until my bus either melts or catches on fire.