Wire connecting to positive battery connection.

keep one battery positive + pole to system red wire...this battery negative - pole to 2nd battery positive+ pole and 2nd Negative - pole to 3rd positive +pole its negative to 4th positive pole and 4th battery negative - to it makes 48 volt and 4th battery negative to system as black
+ - + - + - + -

Starter Solenoid trigger wire loose or bad connection this is the small wire to the solenoid.

best thing to do, as i presume you want a completely new power feed , run a power and earth cable to the battery and connect the power to the positive pole and place a fuse inline at that point and connect the earth wire to the neg pole , leave the fuse out until you have run the cable to where you want it ,this is a much safer way as can't interfere with any other components already fitted keep me updated if you wish

Bad connection at the battery. As soon as large current is demanded by the starter, you lose the connection. Make sure both terminal connections on battery are clean and tight. Now is a good time to do a voltage drop test. Watch youtube videos. Most are quite good at explaining the test. This test is great for finding a bad connection.

This looks like a job for Superman or maybe you! Connect an alligator clip test wire to the chassis of the unit for a good ground. Get a 12 volt battery and hook the other end of this ground wire to its negative pole (-). Hook a second wire to the positive pole of the battery. Put that modular plug back in the unit making sure that all the wires coming out of it are separated. Then, trying the THICKER RED wires first touch each to the positive (+) wire from the battery. When the panel light of the unit is on...you got it! The thinner wires are mostly for the speakers with black for negative an red for positive...the antenna is anybody's guess!

Hi

The black wire will be connected to the body or engine of the van and the red or positive wire is connected to your starter motor.
You can also use a 12 volt battery light to check the negative or positive wires.
You take the 12 volt battery light and place the negative black wire clip to an earth on the vehicle, then when you touch the tip of the 12 volt battery light onto a positive wire the light will light up.
Then you will know which wires are negative or positive sometimes the key has to be on for some wires to work.
Cheers

check the condition of the battery cables

You have a wire going into your starter in the opposite side of the starter. There should be 2 poles where you connect wires. The only one that goes to one pole is the heavy gauge wire coming from your solenoid

Check the ground wire, a black connected to the ( - ) pole, it might have been disconnected. Clean the battery clamp and battery terminals from rust and oxidation, tighten up the TERMINAL. Remember ( + )positive pole is RED and negative ( - ) pole is black, sometimes no plastic cover!
Taaah!

get some jump leads and another battery of car, connect the leads to the terminals after you have loosened them make sure they stay connected while you remove the battery this way the radio will remain powered and you will not have to put in the code