How do I get rid of the nasty buzzzzzz sound from the rear deck? Is it the bose plastic piece that is making the noise? Do I have to tear it apart and apply sound deadening products?

advice from fellow sound gurus is appreciated

 

It's possible that it's the center brake LED plastic molding being pushed up against the rear glass and rattling, such as was the case in my car. The fix was simple in my 07 sedan though as I already had the factory sub out of the deck. I just reached in through the factory sub cutout opening and inserted a rolled up piece of foam directly under the center brake light housing. The rear deck still moves from pressure created by the sub, but is now silent. Of course, my rear deck also has a layer of Dynamat on it, so your results may vary.
Here's a video I took before I had the brake light secured.