[Sergio]

That gray wire with the bullet connector provides switched +48v directly to a pin in the 12 pin connector behind the dash, it does not feed the power back thru the harness to battery compartment.
I think the positive side of the solenoid coil is the best place to tap into the switched +48v if your converter has the trigger wire, meaning very little load placed at the connection point.
For folks that have converters without a trigger wire, they should use a charger relay to switch the power to the converter. The charger relay coil positive connection would them come from the positive side of the main solenoid coil.

Edit: these comments are for newer precedents as the way the main solenoid activates is different for different models/years.