The short way: you go to some horrible store and have some guy do it for money.
The longer way: you buy it online and then install it with your father in-law over the course of about three hours in his garage.
B's dad was an engineer for a big corporate entity for many years. He's in the process of building his second plane from the ground up since retiring. The man knows his way around mechanics, electronics, and combinations thereof. Thus, when we need something fixed that isn't a computer*, we go to him.
It turns out that installing and wiring a car stereo isn't that difficult. It's supposed to be based on standard wiring, in which each color wire corresponds to something specific - yellow is always supposed to be the power, black is the ground, etc. The thing is, the '96 Geo Prizm** has its own made-up colors for wiring. So B's dad looked 'em up online. Then, whoever it was that installed our old stereo introduced a middle tier of wiring between the car end and the stereo end. The result, as indicated in the above diagram we generated over about 45 minutes of tracing wires, was general obfuscation.
That said, after a lot of clipping and snipping and splicing, we got the thing hooked up. We fired it up and it worked on the first try. A party of triumph ensued! This morning, B and I picked up a 7-dollar hot pink cable so we can connect our iPods to it. And now, for the first time in months, we have tunes in our car.
Around my apartment, this counts as big news.
* I can still fix those.
** It is a sexy car.