New Stereo - Negative Speaker wires?

[bertible]

Hi all,
Replacing the orginal BMW stereo in my 87 cab. The old unit had only two positive speaker wires coming out of the head unit, going to the factory fader switch, from which 4 positive wires come out. The negative speaker wires are no where to be seen behind the radio. Does anyone know:

1/ Where the negatives are? (easily accessible?)
2/ If I need to rewire, can 4 separate speakers use a common negative wire?
3/ If Yes to #2, why do all new stereos come with 4X2 positive/negative wires? I would be twisting all 4 negatives together and don't want to ruin the stereo.
4/ If No to #2, why doesn't the factory stereo have the negative wires going to it?

I know the colours of the negative wires, but have not been able to spot them under the dash.

Right Front (-) Gray/Brown
Left Front (-) Gray/Violet
Right Rear (-) Black/Brown
Left Rear (-) Black/Violet

(Same on all E30's I think, I got these colours from the web then verified them on my car)

Thanks for any help.

Tuneless in Vancouver
Gary

[bertible]

Got a reply on the cabrio forum. It turns out that there is a separate amplifier (in the trunk, fwd wall, on a cabrio) and that the 4 outputs from the fader are inputs to the amp. Then all the speakers driven (pos and neg) from there.

Looks like I'll be using separate speaker wires, so I can reinstall the factory system, with amp, in another car.

Gary