If you wait around, I'm sure coastaletech or that other guy will come up with a nice kit. However, if you want to be cool, you'll help out other owners and figure it out yourself, posting instructions the whole way.

If you got the premium sound, you could tap into the audio outputs of the radio before the amplifier, and add some sort of switch to switch between empeg and stock stereo. Or, you can actually just run the audio out of the stock radio into the aux of the empeg, and then send the out of the empeg into the amp. All this would require either just cutting and splicing the wires, or finding the right connectors to whip up some custom Y-cables.

Now, if you have the regular (non-premium) stereo, it's probably a little harder, since the amp is integrated into the head unit. In this case, either you add an outboard amp, run both the stock stereo and the empeg to them and switch between them, or fashion a way to get the empeg sound into the CD changer input of the stock stereo. If you want to tap into the changer input, finding the right wire to connect to won't be a problem, but figuring out how to tell the Prius AVC-LAN that there's a device available on that input is trickier That's what some of those links in this thread have done in their kits, by tacking on a little circuit board that knows how to speak AVC-LAN.

For the physical mounting of the empeg, you shouldn't have any problem putting it in the lower dash storage compartment, after actually taking the storage pocket out. That's where the add on changer goes (for the lower end stereo), and that changer is DIN sized. If you were to buy the changer to add it on after purchase, there might be a "settings kit" required that would include the cable for it and possibly hardware to put the changer in that pocket. Depending on whether that kit is required (like for the 2001-2003), and on what hardware is in it, it may be worthwile to purchase to help mount the empeg in that spot.

Now, in case you haven't found this yet, these are your three best friends for this project:

http://autos.groups.yahoo.com/group/2004-prius
http://autos.groups.yahoo.com/group/prius_technical_Stuff
http://autos.groups.yahoo.com/group/toyota-prius

Now, whatever you do, please report here to keep us all apprised. I'm especially interested, because I have one on order to come in probably next month, and I'll definitely be putting my empeg in. At this point all I know comes from the groups, the various tech documents, and my dad's 2003. Having some hands on reports from someone trying to put an empeg in a 2004 will be priceless.