Installing an LED can be a somewhat cumbersome procedure. Complicating matter further is pickup from one side comes in from a set of wheel back wipers and is fed to the motor through the red wire seen at the top of the engine in the above photo. Power from the other rail however is routed through the frame itself. This makes tapping the power difficult.
One solution is to solder one wire from the LED to the frame. I however, took a somewhat more complicated solution. See, like it's bigger brother, the K4 Pacific, the B6 suffers from poor power pickup. Only four of the six drivers pick up current. I added a set of Bachmann tender trucks and hardwired them to the motor. I tapped these connections to power the headlight.
With the wires in place, I Removed the original headlight tower to make room for the LED and 1000 ohm resistor. Then I used alligator clips to mock up the circuit before soldering. With the engine running forward, I connected the the LED and resistor to the leads to determine the correct polarity. If you hook up the LED and it doesn't light up, reverse the alligator clips. When it does light up, use a sharpie to mark one side of the LED so you remember how to connect it.
I soldered the LED and Resistor as close together as I could because space in the boiler shell is limited. Once I got everything soldered in and positioned, I put the engine on the test track. It lit up nice and bright but light spewed out from around the smoke box. To stop the light leakage, I glued a small piece of styrene to the bottom of the boiler shell.
While I was at it, I also touched up some of the paint and redecaled the engine.