Are you saying both rear channels and both rear speakers were blown??? Wow!
Does your radio look like this?
If so, keep an eye out on ebay. You'll find these out there quite often AND you can usually get it for a decent price. Make sure though that you get the CODE for the radio.
I found one at least one out there:
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=6774&item=2414874979
You can also check here for radio (one of my favorite parts site:
http://car-part.com
Lastly, to repair or exchange the radio (as you probably already know), go here:
http://www.beckerautosound.com/
Willy