@Chris1983 Looks like you’ve got a couple of piggy backs on the CBE already. I only have one wire.
The bottom wire looks like it’s almost falling off too.
If it was me I’d completely rewire them to a new distribution block, but I know electrical isn’t your thing.
This isn’t how I’d do it but should work, if you aren’t confident enough to rewire:
Do you know where your main leisure 40A fuse is located that feeds the CBE block?
Remove the fuse, measure the voltage at the CBE lower terminal to ensure it isn’t powered then pull the 3 wires off the bottom CBE terminal, keeping the group of three connected together.
Crimp your new Solar controller output wire onto your new piggy back spade terminal and connect it to the now empty CBE lower terminal. Now crimp a small length of the same 4mm2 red cable with a single spade terminal on one end and a piggyback spade on the other. Connect the single spade onto the spare of the piggy back that you’ve just connected up the solar wire to and connected to the CBE terminal and then connect the other end with the piggyback onto the group of three that you’ve recently disconnected. All your wires should now be connected together again. Run some cloth insulating tape around the bunch of crimps to keep them together and then support the wiring mechanically with a cable tie and cable tie base so as not to put a mechanical load on the CBE bottom spade from the mass of loose wiring.
For accessing the ground wire I found the best way was to fold the bed down and work facing the rear. You can then kneel and get both hands in and down and should be able to see what you are doing. Socket on a socket drive to remove and reattach the ground stud nut which from memory is right near the back to the side of the tailgate lights. It will have numerous other black wires (or brown) going to it.
Once you’re done you can reinsert the 40A fuse.