Speaker shop 70-80 dollars to rebuild your original plus shipping both ways.
I juast found this place (See link) that reproduces the original GM 4X10 10 ohm speaker. With the design they used you can rotate the magnet cover for multiple mounting brackets for different GM cars. I have one on order it should be here this weekend. I will post picture when I get it.
Jason
http://www.turnswitch.com/speakers.htm