Some cars came with it, some without. You can find it in eBay, on scrap yards, or buy it from Saab. It's a direct replacement, you just need to provide the power for the mirror. Later cars have an accessory wiring harness to plug the mirror in. If the harness is not there, you will need to do the wiring yourself. The only tool required is a Torx/flat screwdriver from the trunk.

If you need to quickly test the mirror, power it up by applying 12V between the red wire (pin 5) and the black wire (pin 8). Then cover the sensor on the back of the mirror and point a flashlight or another source of the bright light at the glass. It takes about 3-4 second to dim, giving the image a greenish hue.

The wiring used in this guide is specified by the Saab manual for early style mirrors with no compass and homelink device. It uses an non-switched power source. Later style mirrors draw more current, so you need a switched power source (e.g. a sunroof lead) to turn the mirror off with the car and prevent it from draining your battery.