If X-12 also has the Charge light wire, that is also where I grounded the coolant sensor wire.
When I installed my Megasquirt I repurposed the charge wire for a fuel pressure sensor.
To wire up the charge light I used the X-12 connector, and ended up with the beeper beeping. At first I thought it was related to the charge light,

which made no sense, but after some searching through the FSM, I figured out it was the coolant beeper.