I installed a bazooka sub in my car the other day and I was playing around tweaking it tonight. I found out something pretty interesting.
the system is bi amped. meaning each speaker has its own amplifier, common to high end installs, maybe it's more common in some higher end oem systems but, this is the most expensive car I have ever had so it's new to me.
I noticed when I panned the speakers all the way forward that my sub was still playing. the highs where clearly off so I looked a little closer under the back dash,
sure enough there is another wire feeding the mid/highs.
I am pulling off all of the panels probably tommorow to install some dynomat, I don't understand why they just don't install it from the factory, I can hear the metal outside the front doors resonating like crazy, it drives me nuts.

(tounge in cheek) Don't worry, after you start loosing your hearing, those metal door panel resonations and highs will no longer be a problem... I learned this the hard way....I'm down to 10% hearing in left ear and 75% in my right all from loud music (nerve damage) when I was younger.