I fought with this a few years back. From what I remember the interior base will slide out from the ends but the metal frame the trucks are attached will drop with a little effort once every hidden screw is out. Unfortunately the frame is screwed to the interior base. It gives you the impression that it can all slide out but in fact the frame drops and the interior slides. You might have to loose some of the detail parts on the bottom to find all the screws. Once the frame is dropped, you slide the interior out holding onto the base so as to not break the wires. I've included a photo of mine with the frame dropped but the interior base still in the car.
This is what you are probably fighting.