welcome. Its a pita but its simple.

-remove rear seat cushions
-undo 4 bolts, 2 big ones for the seat belts and 2 smaller harder to find ones near the bottom edges of the seat
-lift the rest of the seat assembly up and then out (they have hooks on the back)
then you will be able to see the plastic retainers holding the rear deck piece, remove them
-replace speakers
-repeat everything i said in reverse order