You guys read my mind. I just acquired a 2000 Celica and I was wanting to do this myself, if I didn't already have a Mazda 6. So I'm going one step down and installing what I had before the carputer:
Dell Axim X30 w/ iNav's iGuidance software
Garmin eTrex GPS
Sony Mp3 head unit

I've never done work on the Celica. Where can I find a guide or something that walks me through the steps of opening up the Celica and installing a Sony HU?