Remove the ground (green-yellow) wire from the metal finger (red circle).

Next, bend up the little metal pins, marked with green circles. Be extremely careful, it is very easy to damage them! They won't survive more than a few bends. In fact, one broke just after opening and closing it once!

You'll need to bend them a little outward (like the ones marked with a green circle or a little further, the one with a red circle is not bent enough yet)!

Make sure you don't miss any of them. There is a good number. You won't have to touch the ones at the bottom.

Now you should be able to slide the metal carefully down. If you encounter too much resistance, make sure you're not hitched on anything (like the cables) or that you didn't forget any of the metal fingers.

At the side you should find a hole in the plastic that gives you access to the pump and the cables.