FISHBREATH said

Might have been Grade C (Chinese) screws. What size are the screws? I can't imagine anything like a 10-24 breaking like that. The screws must screw into the metal of the trunk lid. They are probably rusted in there, but they screws have been faulty or already broken.

I agree that the best bet is to soak them good over night and then go after them with whatever tool you need to get them out, even if you need to drill out or grind off the heads. If they break, put stronger screws in later. The worst that could happen is having to drill out the broken off shaft and re-tap the hole; not the end of the world. I would use something like big Allen cap screws, such as what the Superbird used on its wing adjustors, when re-assembling.

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