Use the two black wires running to the solenoid. Find which is positive and which is negative. Connect the annode(+) of the LED to the positive wire. Connect the cathode(-) of the LED to a 350 ohm resistor. Connect the other end of the 350 ohm resistor to the negative wire of the solenoid.
This will light the LED each time the solenoid is turned on. The extra current draw is only 20mA This is almost nothing when compared to the Amps of current draw by the solenoid.
It should work fine. Have fun.
Except for the Automag in front, its usually the man behind the equipment that counts.