Oh yeah, I wouldn't post and not give the answer to the puzzle. That doesn't help anybody.
Thanks to all puzzle solvers. I feel a little stupid not figuring out that it was a fusible link. In my defense, the fusible links that I have previously encountered on other vehicles regulated ALL battery power as a last-ditch safety against a catastrophic power discharge.
In summary for any future similar posts, the "Main" fuse controls everything but seats and driving lights.