wiring

Some mis information in your post Paul, I'm afraid.

Your comment about serial DC wiring only makes sense with Christmas lights ( the old fashioned type). Not applicable whatsoever in van or car wiring as every light bulb in the series chain would have to be of a proportionally smaller voltage than the applied 12V. So, lets best forget about that one.

Then, using the multimeters buzzer is fine but you should also mention the warning not to use it on live 12V wires as the meter might not like that. Disconnect the van battery and the plug to the car before poking about with the multimeter set to buzzer. Anybody using a multimeter for the first time should read the instructions, then make very very sure the rotary switch is set to what you they trying to measure and the probe leads are in the correct socket.

Further, I think you are wrong about not needing an electricians licence for 110V or under systems. Extra low voltage goes up to 50V, not 110!

Then your next comments would have been better unmentioned as they only add confusion.
For your information, it is the current that flows through the heart that kills and it needs to be only small to do so.
Where the confusion starts is how that current can flow. Consider your skin is like an electrical resistance that can change if the skin is dry or wet, thick or thin or damaged.
Now, it takes a certain voltage to push a deadly current through the skin, there is no hard and fast figure what it is as it depends on the skin condition. And, the current must flow trough the body, it won't kill if it only flows between two fingers of the same hand. I would not try the latter on voltages over 50V as you never know how well the rest of the body is insulated from earth!

So, you cannot have high amps with low volts UNLESS you also have low resistance in the circuit.
The amount of current (amps) flowing with a high voltage also depends entirely on the resistance in the circuit.

I think it is not practical to 'talk' somebody with limited knowledge through the troubleshooting process on the internet. This is much more successful if demonstrated hands on for a particular application.
For general electrical troubleshooting advice, I would wonder why I did a 4 year electrician's aprenticeship if this knowledge could be dispensed with a few comments in a forum such as this one.
Klaus