The problem you are having with the volume vs amplified system is not a difficult fix. The Parrot puts out a full signal, strong enough to drive an actual speaker. The OEM amp is supposed to be getting RCA level signals (barely enough to drive a small headphone). See the difference? That is why the Parrot is so much louder than the radio....I wouldn't recommend it, though after all is said and done, I enjoy using mine. Here's my rant from the day after I installed it:
http://lean-off.blogspot.com/2009/11/hands-free.html