If you pop the roof off the car, I believe that gives access to the truck mounting screw. If you undo that you should be able to get access to the coupler screw. I think I used some loctite on mine as they had the same problem running on my christmas display. It's been a year or two since I had them apart to know for sure, but I know you get access to the screws by removing the roof. Whether or not you have to pull the interior out, I can't remember (I don't believe that was the case though).
To remove the roof I believe is two screws underneath (you should be able to see the mounting posts for them through the windows to help locate the screws). I believe you have to squeeze in on the edges of the roof as there are lock tabs that hold it to the walls when the screws are out. As I recall, I usually work from one end to the other to release those tabs and remove the roof.
