i dont know for certain if your car has the bose amp or not, but that would be the first place i would look (especially if that amp is located in the trunk area).
if it's just one speaker i would just check that one particular location, but if it's all around, then it's probably happening at the amp.
the crackling/sparking noise doesn't usually happen pre-amp (if line level it usually just cuts out or makes scratchy sounds) so probably not at the headunit.
i could be wrong, i donno, since i can't diagnose your car in person over the internet.
you'd be amazed how wet these speakers can get inside the doors.
