The. Solenoids attached just below the PS battery are for the grid heaters and like most solenoids are attached to positive voltage.
If there is some sort of solenoid attached to your ground cable it must have been installed aftermarket
It breaks the ground, and I am having trouble with my grid heater, DTC P2609
Quick question ... Are you the trucks original purchasing owner ?
Well that top picture certainly looks like the grid heater/battery solenoid.
Why it seems to you that is controlling a ground wire instead of positive voltage is a mystery me....someone either messed with it or you need to go look and verify the wire again.
He is talking of the large green wire on the large contact nut. The small wired go to the ECM to energize the contactor.