Diagnose electrical issues

I believe I have two related electrical problems, the second one likely self inflicted by trying to work around the first one.

Part 1: Went to pick up the TT from storage today, installed the battery (fully charged), tried the tongue jack and nothing. Connected to the TV power and still nothing at the jack. Brakes and lights on TT work.

Connected to shore power and lights and appliances in TT work, but jack is still dead. Tested jack fuse and found it is good.

After deliberating, I noticed and jumpered this, which is mounted on the frame right behind the battery box and is connected to the battery terminal:



Whatever this is, the jack worked while the two posts were jumpered. I hitched up, removed the jumper, and came home.

Part 2: Got home, re-connected jumper, expected jack to work and instead, got nothing. Connected to shore power, still no jack, and now only appliances work in TT, not lights as previously before I did the jumpering.

My guess is whatever the part in the pic is, it was wrecked at the outset of my day, and that by jumpering it I wrecked something else in the line somewhere. Any suggestions on where to go looking from here?

Thanks!!

DougT