The Scoshe FM-MOD01 blocks out the regular antenna signal (from what the guy at Crutchfield said) via a switch.
I see we're thinking alike because I asked about an FM booster. His response was with the Scosche Mod01 no booster is needed because it operates at the full range legally allowed for an FM signal to be transmitted.
And again, this is just what I was told but he said the system actually can move the signal from the ipod at a better quality but the system has to compress the signal to stay within regulations almost like taking a music file and compressing it into an mp3 just for the sake of size.
I don't like that at all but again, it's better than the cassette option. It gets so bad at times I want to just put on my headphones and drive...
I did find the scosche mod01 cheaper when I did a search but I dig the crutchfield customer service.
Also, if you are planning on buying any additional stuff,
http://dealnews.com/coupons/ca...ec=21
there are coupons on there for crutchfield.