Audio Systems: radio volume fading in and out, pioneer cd player, car stereo installs


Question
I just replaced my old pioneer cd player with the newest one out because i was having problems and we thought it was the unit but im still having the same problem..
PROBLEM:
No matter what speed or rpm my radio will fade in and out, it will work fine one minute then the next the volume goes super low and then back up..like i said its a new unit, new rca's..and its not the speed volume thing that comes on the unit where if your going slower it goes down in volume cause thats turned off, and it does it at no matter what speed im going... could this be a problem with my amp or something..me and my husband are lost and he has been doing car stereo installs for years, and has electronics backround but we are clueless..he plans on changing the ground out because it might have come loose..but this problem,,has been occuring for a long time even before that.That just happened.
any suggestions would help, hope this makes sense

Answer
Does it happen on all sources or just one? (CD, XM, AM, FM, MP3, etc)
How many speakers is the head unit's internal amp driving?
(4 is fine, more than that could be trouble)
What kind of speakers is the amp driving? (Subs, separates, stock..)
My first guess would be a blown speaker, shorting out the voice coil, or speaker wires shorting to each other, or ground.