In my 2005 chevy impala ls its all stock sound equiptment and the sound is cutting out and not working !?

help!! my 2005 chevy impala ls has all stock head unit, stock speakers, and stock amp, when i plug my back speakers in while im listining to music the sound will simply stop or become extremly distorted. also when i put my key in when the door is open the door chime becomes distorted too. what could this be?! please please help asap!