Btw: Just to note: there is a TSB out for this audio issue. They'll fix it with a better ground loop isolator for free.
i have a sirius radio i put in my 08 prius. i hooked it up through the aux input that is in the center console. it has a horrible hiss to it. driving me crazy. i have the same sirius radios in my 08 tundra, 06 acura tl and my car that i drive at work and have no problems, sound is great. i bought a direct connect (attactched directly to the antennae). i was going to have toyota pull the radio and hook it up for me, but i may wait since it sounds like this may not help.