Bad crimp on the wire termination in the diode socket?

You can get to the bottom side of that fuse block. There's a snap-on plastic cover that covers the input feed lug on the front side.

Slide that cover off and remove the nut from the lug. (Probably should remove both ground cables from both batteries first.)

After the lug is removed from that feed, there are (I think) two snaps on each of two sides of the block that will allow it to come up and then there are three clips that hold the bottom cover on. You can then turn it over to reveal the underside wiring.

EDIT: From an article I did a few years ago:
"The easiest way to access the bottom of the fuse box is to start in the front. Pry up the plastic cover that protects the main (heavy guage) power input wire, and set it aside. Remove the nut (it will be very tight) that holds this wire's terminal to the fuse box. Pry the two little clips on each side of the box that hold it to the support bracket, and lift up. Three more clips hold the bottom cover onto the box. Remove the bottom and gently turn the box at least 90 degrees so the underside faces you."

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