Look at the positive (red) side of the solenoid where the black wire with red collar is attached.
That looks like a charger pigtail wire. Looking at the black wire held in hand it has the same type collar except black instead of red.
It was that particular style collar I noticed which is the same type used by Deltran battery tenders.
I don’t know if that location is unusual for factory or dealership installed charging wires. I added the one on my Scout myself, connecting directly to the battery.
i’m going to guess it was connected to your battery since you say (oops! your wife) you removed and wife installed it.
So I doubt it was connected to the frame....
Unless (she {oops again}) you began the removal by disconnecting the frame ground wires.