You should purchase a wiring harness for your car. It'll make it lot easier for installation. It can be purchased from most car audio shops for around $15. Here's a wiring diagram for 89 GM cars.
A Right Rear - : Light Blue B Left Rear - : Yellow C Right Rear + : Dark Blue D Left Rear + : Brown E Left Front + : Tan F Right Front + : Light Green G Left Front - : Light Gray H Right Front - : Dark Green I Power Antenna Turn on: Pink J 12 pos ignition wire: Yellow K Headlight Dimmer wire: Gray L Ground Wire : Black M 12v Battery wire : Orange