@Rando: Nice. I'm just about to start building a similar system into a Celica GT4 using an SP7 and an M10k, and an Opus, in the other thread, you mentioned using relays, so I dug this out. Very Sweet.
@Keggerman: I hope to be able to get everything into the double din space behind the screen if I lose the cupholders. I using sheet steel for the case (1mm bought sheet, and 2mm for the sides from the from off a grill from an old cooker that needs to be dismantled to go to the dump) should be rigid enough and allow more space than using MDF.
@Rando: I like your thinking on using the harness. I'm planning similar as the last owner of the car supplied me with the Toyota->ISO adaptor that I can hack into for speaker -> AMP -> PC, and for things like ACC and amp/antenna remote lines.