Maybe I have miss understood you.
But what you said makes no sense, unless the sound is only coming from left or right channel? Not the front or rear. If this is what is happening then you must have your channels miss wired.
Check the balance and fader from your stereo. And make sure that you have the amp connected correctly.
If not wire them up right.
Check the balance in your volume control panel, pay close attention to the line in.
Then check the cable from your XM and make sure that you donít have a bad cable or that it is not partway unplugged.