My 2008 XKR Coupe's rear speakers sound very strange, kind of like a weak signal, but the fronts are clear and strong on the same station. Rears are fine on CD.

 

It works properly with CD's
But with radio its weak. I am guessing All-- AM, FM, SAT.


To me sound like the radio. Inside the radio that feeds that speaker.
If it was a defective speaker, the problem would be all services, no matter what you are tuned to.

 

It gets a little worse depending on whether you have it on stereo, 3ch or surround. Its like whatever is involved it changing the sound has a defect that only effects the rears.