What I'm goin do?

[Skunksmash]

Well for some reason, the guy who painted my truck broke the door striker plastic. Don't know how, other than he was dumb. So the other day I went to replace it, and you can see the results in the pics. The nut that it goes into has fallen down inside the cab. I took the speaker out looking for it, but I couldn't find it. I guess maybe it got down there too far. I may try again to find it, but what if I can't?

How should I go about completing this repair, if I can't find that original nut? It somehow fits into that square hole perfectly. So another random nut from the hardware store won't cut it.