Just following up... once the ammeter is removed from the circuit, doesn't the ignition (key switch) need to be reconnected to positive feed somewhere?
IF you want to install a voltmeter.
There was no need to remove the red #10 wire, pin 5 of the connector, which goes to the ammeter. Move it from the ammeter to the switch. Presently there is a red wire jumping from the ammeter to the switch which goes on the voltmeter positive stud. The negative stud gets connected to the black wire from the ammeter light connection.
All risk of burning up the harness disappears when you disconnect the orange wire. An in-line 30 amp fuse should be added to the red wire near the solenoid to protect the engine circuit.
It is important to note that the ammeter is not the problem with this circuit. If your panel does not have an ammeter the harness still has a problem. The problem is running the alternator output to the engine panel and then back to the batteries.
If you just disconnect the orange wire at the alternator (and jumper to the solenoid or house bank) you do not have to touch the engine panel at all. The ammeter and all of the other wiring will work fine. You should only replace the ammeter if you want to install the voltmeter. The ammeter will read the current used by the engine - glow plugs, fuel pump, instruments, and lights. The needle will barely move, however.
As always, YMMV.