I'd be looking at continuity of the grey wire, in particular. The problem would seem to be somewhere in your wiring harness. Have you tried using a temporary duplicate wire i.e. wired up something outside the factory harness?
P1246 - Generator Load Input Failed
The PCM monitors generator load from the generator/regulator in the form of frequency. The frequency range is determined by the temperature of the voltage regulator where 97% represents full load, below 6% means no load.
Generator circuit short to GND
Generator circuit short to PWR
Generator circuit open
Generator drive mechanism
Damaged generator/regulator assembly
Verify battery voltage is 14.5V.
Verify generator/regulator has the correct part number.