Mark....I did this myself only recently and all I did was remove the 3 screws (one in the door handle and 2in the grab handle), then I got a flat blade knife and just worked the door card loose. You just need to slide something behind the door card and apply some force. Don't worry you wont break anything.
My disco is a 2003model, so if yours is the same then that's all you need to do. You don't need to remove speakers or anything....you will see the cables connecting them once you remove the door card....just disconnect the cables.
Putting the card back on is just a case of lining up the plastic screws on the door card with the holes in the door and hitting the card (hard) to "pop" it back in one at a time.