A very simple Google search for "toyota prius stereo" turned up this link. It looks like it was made for the 2001-03 models, but it may work for the 2004. If that adapter works on the same IR frequency as the Empeg, I'm sure you could use the built in controls of the Prius to control the Empeg with ir-translate in Hijack. You should also join the 2004 Prius Yahoo Group and ask about aftermarket stereos.