If it has working ditch lights on the front deck, the wires to them pass between the shell and the front of the chassis, through shallow channnels. They use a small piece of sticky black tape to keep them in place on the chassis to prevent them wandering during factory assembly. The problem is, it also sticks the whole damn lot together. Just keep wriggling and working the back part up and down to loosen it. A right PIA when you don't know what they've done. Once the grip has broken, level the chassis and pull it down evenly. It it was assembled too soon after painting, the shell may also drag on the sides where the paint has stuck a bit (had that twice on SD40's)