Sorry, i did not take any pictures while taking it apart. But, let me see if i can remember.
First take out the 4 screws that you can see. Next i believe i used a small flat head screw driver and pried around the plastic part where the button is. Now when you do this it will probably brake the plastic clips but thats ok, i used a dab of super glue to keep them together after testing and putting back together. After you get in you are on your own, i forgot at this point what i did exactly.
Just use a toothbrush and alcohol and try and clean the corrosion from the water the best you can. If you get stuck send me a pic and i can see if it helps me remember what i did.