to add to what brewski said
remember the MIB controls the circuit by switching the negative to the contactor on, so when you check the voltage it should be there until the contactor is switched on, then it will drop to 0 volts or near 0.
Also i don't currently have a wiring schematic for this truck handy but i think on some trucks the steering circuit was triggered by a switch on the foot pedal that supplied a signal to the MIB telling it that someone was on the machine and the pedal was pressed so it completed the signal to steering to allow it to turn on the contactor to activate the steering motor, earlier models just turned it on when the key switch was turned on, pre MIB machines.

Since i do not have my manual on this machine i'm just making an educated guess from memory. ;o)