There are generally 2 ways to do this.

1)  Assuming that this is the (proprietary) factory harness that
plugs in to the OEM head unit:  There are a few accessory manufacturers
that make an adapter that plugs in to the factory harness, which you
then wire to the harness that came with your head unit.  This adapter
harness will have all wires labeled, as will your after market head
head unit's harness....  So connect the wires.  These usually aren't
too expensive; and I believe that Crutchfield even includes these for
free with a head unit when you purchase.

2a)  Do it the old fashioned way.  Installers usually have a book that
identifies the wire colors used for battery/ignition/ground/speakers
for each car for each model year.  If you can get your hands on this
information, you can use it to identify which wires are which, cut the
OEM wiring harness off, and then just splice the after market head unit
wires directly.

2b)  Do it the *really* old fashioned way.  If you can't get access to
the wire color chart, it's possible to determine which wires are which
by probing and using a multimeter or test light and continuity checker
to find constant and switched power, ground and speaker leads.  A 1.5V
battery is also helpful when finding speaker leads to identify each
speaker.  This is a pain, but it's OLD SCHOOL, baby.  ;-)

Good luck,

Hey, I bought a Geo Metro once!

Well... I didn't actually BUY it... I got it in a Happy Meal...

Tried an auto wrecker?

You should be able to find a connector to plug into that harness, that will have
color-coded wires to connect to a new deck.  If a local stereo shop doesn't have
it, Crutchfield will:
http://www.crutchfield.com/S-Y2K9PULTktL/cgi-bin/ProdView.asp?g=103000&I=120701782

"Receiver Wire Harness
For select 1985-up Chevrolet, Geo, Isuzu, Pontiac, and Suzuki models"