Hi - probably admin might suggest not putting your e:mail in a post because searchbots might find it and start spamming you. You might want to edit post to remove it.
I am sure someone will say junk yard is a suggestion but I looked up part number from GM and think a new speaker would be either #16181962 (UQ3 option) $30.44 or #16154031 (UX7 option) $14.29
Since you have a GTP, I bet it more expensive one but check option listing on your vehicle which is usually on underside of trunk (if you don't have original window sticker still).
Oh those prices came from
http://www.gmpartsdirect.com/
Course I don't know how hard to actually replace one in your vehicle.