Door speaker squealing?
 

Hello, I recently became the owner of my first Volvo, a 2006 S40 2.4i, I'm completely sold on the car, the company, and this site. But I've unfortunately run into a problem that I haven't seen on here yet...

Since I purchased the car I haven't had sound coming out of the drivers door, so finally today I pulled the door panel off and found the problem! Both the tweeter up top and the main speaker below weren't even plugged in! **** yeah! problem solved right?! WRONG!...

I plugged the speakers in turned the ignition on and the speakers started screaming, squealing, and giving all kinds of strange feedback, turn the stereo off, pulled the key out, and they still kept going... Finally as I was re-removing the door panel they stopped. I gave it a second test with the panel off, and it pulled the same crap, I unplugged them both and left them. So my question is, WTF is wrong with it?! It's only on the drivers door, every other speaker in the car sounds fine. It's secluded to that door, and all the wiring looks completely unmolested!

Theories? Anyone else have a similar problem?

Any feedback is appreciated! Also, It's a premium stereo if that makes a difference.