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No, don't change anything, including wires. Take some measurements.

The crucial ones are :

1) Between battery + terminal and starter + terminal while cranking.

2) Between battery - terminal and the starter motor body, while cranking.

In a good installation, both these voltage drops should be minimal - less than 1V. If they are greater, you can start narrowing the culprit down.
 

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Nothing wrong with replacing the trailer connection, in fact it's worth doing. But it won't fix the problem you reported, which as I understand it is, you hear the solenoid click, you can see the engine trying to turn over but it does not, and you have 12.5V at the panel while cranking (this was while cranking, wasn't it?) This set of symptoms cannot be caused by the trailer connector.

What the trailer connector can cause is low or no voltage to the starter solenoid. If this was the case, it would not click.

I say again, what you need to do is take some voltage measurements when the fault is present, WHILE CRANKING. I'd like to add voltage at the starter solenoid to the list.

I say this as I spent the last year trying to fix the same symptom.

Problems I found :

1) Battery cables undersized and poorly crimped
2) Loose connections in the battery cables
3) Temperamental and resistive 1-2-both switch
4) Solenoid signal : Ignition switch dodgy, trailer connector dodgy, loose connections
5) Starter battery getting weak

With new battery cables, ignition switch, 1-2-both switch, starter battery, the fault was still present. Replaced the starter motor, end of problem.

It doesn't half start nicely now, though! You just touch that button and vrooom.
 

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The main starter current doesn't go through the ammeter, it is wired (almost) directly to the starter via the 1-2-both switch.

I think what MaineSail was proposing that you are referring to was to take the 1-2-both switch out of the starter circuit.

The things that go through the ammeter that are significant to this fault (as designed by Universal) are the glow plugs, the alternator charging voltage, and the starter solenoid.

If the starter solenoid is clicking, that eliminates the trailer connector as the problem, as the only things that matter when cranking are the starter solenoid voltage, which we now is OK because the solenoid clicks, and the voltage to the main windings for the starter, which are wired directly to the battery (or almost directly, the 1-2-both switch is inline).
 
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