The silk screen on the top of the connector shows HTR/GND/3rd Rail. From what I can remember the 4th pin (closest to the center of the PCB (opposite 3rd) is serial data from the R2LC.
So, from the square pin you have 3rd rail (center rail collector, 18V), GND (chassis ground), HTR (this is the element lead from an AC Regulator (likely a 691ACRGE01) which itself has 5 wire connections we'll get into next), Serial (pin 24 from the R2LC).
(the fan motor will be on even when the heating element is off (hence 3rd rail and ground, constant power) the fan motor pulses based on chuff commands on the serial line. The fan motor will not run in idle or neutral. This is how the unit puffs in sync with the chuffing.
The AC Reg, it has 5 connections (again marked on opposite side of where the wires go): 3rd rail, chassis ground, heating element hot, element ground (no need to connect this lead), serial (pin 24 on the R2LC). The AC reg runs the 6 Ohm element at a constant voltage, regardless of track power in command or conventional. It uses serial commands to turn the element on and off (AUX1 8 and AUX1 9).
Knowing all of this you most likely no longer want to bench test it! (sorry).
Thanks,
Mike