Nice Ertl. I bought one for $5 (new) yrs ago. With some quick mods you CAN make the trunk operable.
As I recall, the truck (hatchback) was sealed (riveted shut). Disassemble the model, drill out the rivets, then grind away those unwanted diecast tabs. As I remember it, that's all there was to it. The upper hinge assembly (roof mounted) was already in place. EZ-peazy!