Batery box wiring

Big cables or little white wires?

That sounds like someone added those wires because the only additional feeds off a battery is the ECM hot.

That said any little wires are B+ feeds. If you want to double check just ohm the disconnected wire to chassis ground. Someone not knowledgeable might run a CB ground to the batteries or something. So ohm it out to be sure. The whole chassis and engine block are ground planes. There's no need to go to the batteries with a ground.

As far as hooking the batteries in parallel all the positives go together and all the negatives go together.

 

Two are positive. You can follow them up to the firewall. The other is negative.

 

I'm sure they done it right. If I saw 3 same color wires hooking up different I would take special note how they went.

It is strange though. You'd think they'd all be hots. Kind of defeats color coding.

 

Freightliner uses number coding on all circuits instead of colors. I just can't remember the ground wire circuit number in the battery box. Let me rephrase a little, the last ones I worked on were set up like this. I can't really say for about 03 or so and up.