Electrical

Car doesn’t start. Wing mirrors will not cycle. Various fault messages shown on dash. Voltage drop on the fuse box above battery. Right hand side (looking from the hood towards the car) reads battery spec voltage. Left hand side (that runs through a fuse on the front of the panel) shows 2VDC.
When shorted across the two terminals (right to left) + to + on the fuse panel, car starts, but ceases to operate once the short is removed. Built in fuse suspected.
I’ve hot wired the two sides together with a line containing a 25amp fuse. What systems are contained on the left side of the panel - I see one heavy wire, what is it?