First, to remove the "removeable piece", use a flat-head screwdriver to depress catch 1.

Then, using the same screwdriver, wedge it gently in the small gap and pull out catch 2.

The removeable piece will pop out fairly easy.

In your situation, there will be cables all over. DON'T pull them out of the connector; just shift them aside.

Picture 3 shows the left wall without the removeable piece. Take note of the position of Catch A.