You can upgrade your speakers... I did with Alpines. You will need this:
http://www.installdr.com/harnesses/gm-wiring.asp
And these adapters for the front door speakers"
http://www.amazon.com/Metra-82-3010-.../dp/B0099Z8KNG
Your rear speaker replacement brackets will have to be custom made a custom shop or you can DIY. No one makes them