3. The old fashion way:
a) Find the icon in the Finder, not the dock to change it.
b) Do a "Get Info" (command button+i)
c) In the top left corner of the 'Get Info' window you will see a small icon for the icon, this is what you will change.
d) Make sure you already copied (command button+c) the icon you want. Then in the top left 'Get Info' window do a "paste" (command button+v) to change the icon.
e) To change the icon back to system default just do a 'Get Info' again and in the top left hand window just tap the "delete" button.
Now I am not sure if it will help you. However once you find the two sets of Trash icons you realize that for the Trash Icon has two sets of icons (for open=FullTrashIcon.icns) (for closed=TrashIcon.icns) that have four sets of different stages of the Trash folder (at different sizes). So changing the Trash icon will takes some time to do by hand.
So I say again using CandyBar and getting CandyBar icons from the IconFactory will be your easiest path.