Bought a used cargo, previous owner was electrical engineer, no idea what to remove and leave

The + connection(s), typically red wires , for that stuff is probably ultimately connected to a post on the side of the seat or to the positive of the battery, which is under the seat (4 bolts to remove seat)
The - connection(s), typically black are either to something metal on the the van chassis somewhere or the negative terminal of the battery.
There may be a small gage wire(s) like the one from the solenoid (cylindrical silver thingie) the ties to a connector under the seat.
Basically trace all that stuff in the photo back to it's source and remove. First thing you should do before messing with any of that is disconnect the battery negative and put that cable where it can't touch any other metal. You will need to take the seat off to do that.
If you post photos of the wiring under the seat while you are there it will help folks guide you. Also take a shot of any wiring to the seat. That will also help.
Something that you can easily do yourself ...assuming the EE did it right, there was no need to "splice in" they should just be additional connections.

If you elect to have someone do the work, IMO it shouldn't take anymore than an hour ...and that is generous.
I'd suggest you do it yourself, cuz it's not difficult and a good way to learn about the battery connections in your van.

FWIW, it looks like what you have is an inverter that was run of a now removed auxiliary battery that was connected/isolated from the van's battery by the solenoid that I mentioned above. If you were to clean up the wiring and add a deep cycle battery with the same chemistry as the one other the seat (either AGM or sealed lead acid) you would have some AC power in the van if that is of any use to you. What are your plans for the van Stan?

Thank you both so much, especially nvjabond for such a detailed response. Saved me a panic attack and visit to the electrician. I fiddled around like you recommended.

njvagabond said:

The + connection(s), typically red wires , for that stuff is probably ultimately connected to a post on the side of the seat or to the positive of the battery, which is under the seat (4 bolts to remove seat)
The - connection(s), typically black are either to something metal on the the van chassis somewhere or the negative terminal of the battery.

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Yup! Here's what I found:

Hey @njvagabond, I retook some photos of the dual battery

first battery (standing from behind driver seat)




second:

Also here are the extra wiring that goes from the back of the van to the front:












In this last one: the wires go behind the passenger dash/front panel, and at a certain point joins with "wiring harness" I believe its called?




Should I just hire an electrician at this point?

josborne1234 said:

Also here are the extra wiring that goes from the back of the van to the front:

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You'll need to find out what connector they're going into. There should only be a few options, a few 6-pin connectors and the 43-pin HSVIC. If it's any of the 6-pins, you're fine just disconnecting the whole thing. 43-pin is a little trickier as the canbus wires still need to be looped. New vans were supplied with a blank plug; maybe the guy saved it?

josborne1234 said:

Should I just hire an electrician at this point?

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I wouldn't, the previous owner did a lot of things the right way. You've already cleaned up the battery box. Most likely these cables running to the back are all positives of some kind (vBatt, vPower, or Upfitter). Worst case, just cut them and tape the ends up.

njvagabond said:

I am not seeing a bridge between the 2 battery negatives.

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It's sneaky. It dips down between the battery and the plastic box. But yes, the latest pics show the battery area pretty much stock now.



This was literally my van yesterday. Getting the winch power done properly, adding a negative busbar, and it's hard to see, but on the inside above the external mega fuse holder is a SmartShunt.

sYfte said:

It's sneaky. It dips down between the battery and the plastic box. But yes, the latest pics show the battery area pretty much stock now.

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Thanks. Now I see it. Yup sneaky right angle lug. See I was right ... OP should defer to you.

You project looks like more fun that mine. My '04 Acura seem to need a new starter. I think I've gone through of all the diagnostics I can to rule out everything else in the circuit (I have no FSM/electrical drawing) , before I go through the replacement ... which requires removing the intake manifold (with one blind lower bolt) ... and all the stuff in the way of that. At least thanks to a really good YT video I found I know what to expect. But not being the most electrically proficient ... there is always some doubt.

Funny story? As I am checking battery voltage ...all of a sudden, zilch. Doesn't make sense. So I figure maybe my newish cheap "fancy" clamp meter must be NG. So I grab a 9V to confirm it's not something with the car. Nope, meter still NG. I grab my old faithful really cheap meter and the readings on the car are as expected. Go back to the new meter, when I realized one probe was not fully seated. Like I said ... not the most electrically proficient.