Project Bulldog. (1988 Diplomat AHB)

[Mikes5thAve]

Use vice grips on it if a socket no longer works.
Hit up a junk yard for a matching spare if you don't line the look of a normal nut (I wouldnt).

 

[AMC Diplomat]

For if you break the one you have or if it's already broken: Pardon Our Interruption...

 

[LSM360]

I don't see a headlight flasher module where I usually see them mounted (rad support in front of battery) but maybe it had one. Wondering if some of your wires went to that.