I have the ML package. And the back left speaker on the rear deck has blown. Where can I get a replacement?

 

The deck only has one speaker on the 2IS: the subwoofer.

If you want a drop-in replacement, you'll have to get a used one from somewhere (eBay) or brand new from your dealer. Alternatively, you can go the aftermarket route, or you can repair your subwoofer. The repair option may be cheaper if they can successfully do it, most actual audiophile places can do it or you can replace the foam yourself if that's the only issue with it.