Good evening all,

Today I decided to install my Alpine headunit into the 300D. I knew about the fader dial, so I snipped that and found the connections and got the connections working for both left front and back via the shared ground L and the Front and back connections. So while I do have great sound coming from the left side, I'm not sure if tying the grounds from the HU to the common ground is a great idea. The HU was running pretty hot, and I didn't want to burn it up.

In theory, the common ground I found would be the ground, and lets say the speakers are the normal 8 ohm impedance speakers, having the common ground wouldn't matter from a electrical sense, because each speaker (the back and front) is sharing the common ground and the + is tying to each speaker.

If this isn't correct thinking, let me know..I know the HU fader works, so I found something correct in the wiring bundle.

The old radio connected only the front left and right, and I found the ground from there and found the back and front for each side from the fader harness.

Slowly but surely, the car is coming back together. Still need steering stuff and shocks and IP/chain checks, but so far it works (i feel it could be faster though)

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Java Developer/Linux SysAdmin

Current:
*1984 300D ~200K,1989 MR2 250K, 2012 Ford Fusion 4.5K (fiance's car to replace the uber-unsafe cavalier)
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Past: 1998 Chevy Cavalier ,2005 Saab 9-3 2.0T, 1996 Mazda Protege, 1974 Porsche 914 2.0, 1997 Land Rover Discovery 4.0L/5Speed, 1995 Jeep Wrangler

The easiest way to eliminate this problem is to trace the wires back just a bit further and unplug the rear speakers from the fader harness. There is enough slack in the wires to run the separate left and right wires to the HU.

http://www.peachparts.com/shopforum/car-audio-multimedia/168194-w123-speaker-wiring-myths-debunked-diagram-bypassing-fader-3.html

The OE speakers are 4 ohm, I believe.

Sharing the - connection is anecdotally attributed to burning up newer HU's due to the increased power. The wiring isn't a true parallel setup of the speakers, but eliminating the fader affects the resistance that the HUs will see.

Or if you can find the solder joints in the harness, you could snip the rear "leg" and pull those two black wires forward. You can unplug and ID the front and rear + wires from the fader mechanism.