Just found this (must remember to use the search button first)
Step 1> Remove the air vents, The air directional bits just pop-out and then you will see two screws up the top which you can undo and their out, Just makes life easier to get to the wires even through it won't expose much access.
Step 2> The speaker is not screwed in or glued down, the way they are held in place is by two clips on each speaker them come out with the speaker cover and shroud. You have to CAREFULLY prise eachside of the speaker out going back and forth until they start to come out and just keep doing this and eventually they will pop out. It does require some force but don't over do it or you will destroy your dash or the speaker shroud.
Un-clip the wires and your don't, It just take patience and lots of beers!!! I replace mine with Fusion 3" speakers. They are NOT 4" they are only 3" too.
Also you might like to see this diagram I made
http://forums.justcommodores.com.au/showthread.php?t=5362