Hmmm....Toyota has been pretty good about using a standard connector set between models. I've seen several Scion HU swaps (I think they're all panasonic/clarion units).
For bracketry, most likely you'll just unbolt the ones off your existing HU and bolt them on the new one. I don't know if there's a "standard" though with regards to what holes are existing in the HU and their location. You might have to modify your existing brackets a bit but that should be pretty simple since you've got the two to compare (take lots of pics/pictures before you remove the old one.
I don't think connectors and mounting will be your biggest headache but getting the trim right. Auto manufacturers like to make the dash area kind of curved, especially on newer autos. So getting the new HU to sit at just the right depth so you don't have some gaps might be tough. I haven't done much car audio in several years though so hopefully some of the other folks will chime in.
You might try looking around on some of the Scion forums. Those guys seem to like swapping things around much more than your average T4R guy.
