If the whine increases in pitch with the revs of the car, then it's more than likely to be a poor earthing somewhere in the system. Do you have any amps beyond the in-built one in the radio?
If whining doesn't increase with revs, then i am unsure!
Yeah, probably due to the earthing of the amp.
Sounds like altenator whine - as that spins up with the revs of the car, it gives more of a whining noise!
Just check the earth - make a better/cleaner connection if possible. That should sort it!