Hey there fish food. I just ran across your thread here. I hope I'm not too late. I see it's been a couple weeks since you asked about it.
I just installed one in my 03' and it is really easy. Just pop off the outer edge of the dash around the factory stereo. I used a knife with a rag around it (so as to not scratch it). Then there are 6 bolts you have to take out facing you when you look at the unit. The stereo should pull right out (just be carefull not to drop the bolts because they may be lost forever) Obviously you will pull the harness and boom, it's out.
If you are replacing the old one with a "double DIN" stereo, or same size as whats in there, all you have to do is swap the two (as far as mounting goes). If you are installing a single DIN, or normal size stereo, you will need a vehicle specific mounting bracket/ filler thingamabob, like this one.
Toyota In-dash Receiver Kit Fits select 2000-up models, chassis size E at Crutchfield.com It looks complicated but it's not. The peices are all labled, and you will have extra's (for other vehicles).
Then you will need a wiring harness, also vehicle specific, like this one:
Receiver Wire Harness For select Geo, Lexus, Scion, and Toyota models, 1987-up at Crutchfield.com Now your new stereo will also have a wiring harness, so what you need to do is attach the 2 harnesses. The absolute best way to do it is to solder them together. This gets the best connection and will actually improve the sound. (Kind of like a water hose, if it's kinked or has holes on it, the water will not flow properly, so it is with the signal). Some people use crimp on connectors, but they aren't a great connection. If you don't know how to solder, learn how, or find someone who does know how.
Finally you can put the thing in the dash and enjoy your new stereo. Now mind you I am a novice stereo installer. This was my second one in my life, my first was in high school about 9 years ago. I will get some pics up in the next day or 2 so you can see the final product. And if anyone would like to dispute, discredit, or challenge anything I have said here, go ahead. Like I said, I am a novice. But this should take care of it, at least it did for me.