Hurdle #3: the aftermarket power tap wasn't designed for this negative post. Needed a separate nut to secure the wire.

Home Depot trip #2. Felt pretty confident it was a #12-24 coarse thread.

Narrator: The nut didn't fit.

Started gearing up for Home Depot trip #3 (😡), but then realized that the nuts that secure the trunk's bottom cover were probably the same thread. Score! That molded plastic bottom doesn't need all four nuts anyway!