'91 CQ radio wiring ?

[Dave]

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Hi,
New to the list.  I am installing a new headunit and speakers using the fac=
tory harnesses.  I am getting very loud alternator noise from the rear spea=
kers only.  The harness that Cruthfield sent does not make sense, they say =
the car has a 'floating ground' and show only 1 neg wire for the rear speak=
ers.  Does anyone have a wiring diagram for the three factory plugs.

THANKS

Dave
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