The real harness you need is attached to the tuner/amp unit, which can be
found under the dash near accelerator pedal.  The leads from the cassette
player and the radio control unit plug in the bottom of it; the power and
ground wires and speaker outputs come out the top.  The antenna connection
is found there too.

An aftermarket wire harness is strongly reccomended, if only because the
factory plug is buried deep in the dash and it would be very difficult to
cut it and make your connections in there.  It's the same harness that you
find in most GM vehicles after 1990.  You'll also need an adapter for the
antenna plug.