Hello stoogieprezi,
You should have gotten a plug-in harness with your new Alpine. To avoid cutting or tapping the wires on your vehicle, you need to purchase an aftermarket wiring harness. It appears that the
Metra 70-1782 is the correct one for the 1990 Prizm.
Then you simply match up the wires on your Alpine harness with the Metra harness. The wiring color code for the harness is shown on the bottom left of the "description" tab and matches up with the Alpine color code on page 18 of your owners manual.
Hope this helps.
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