For Graham Farish N gauge models that already have lighting as installed in the factory, you can usually take advantage of that
lighting with a simple decoder, by plugging the decoder in and using the DCC controller's F0 (lighting) button.
However, if you took the plunge and decided to follow the guide to fitting
DCC sound into your model, then you probably had to remove
the existing factory circuit board to get there. This means that your factory-fit lights will not be connected to anything!
This guide explains how you can directly wire your factory lights to the decoder you installed earlier.
The first step is to de-solder the lighting board from the factory circuit board (or just snip the wires off!).
Next, you need to identify which wire does what on the lighting board. You can use a standard DC power supply with crocodile clips
for this, but be careful
to ALWAYS put a resistor between the positive terminal of the power supply and any connection you make to the lighting board, or you'll
blow the LEDs on the board!
Methodically test each combination of pairs of wires to identify which ones are +ve and which are -ve, and which relate to which
LED colour. Most lighting boards have enough connections to control 'forward' lights and 'reverse' lights, although some have additional
LEDs on top of this too. Most likely is that you'll have a common positive wire which is for all LEDs, then 2 separate -ve wires for
forward lights vs reverse lights.
If the above wiring is true, then the connections to your decoder are very simple... connect the positive wire via a resistor to the BLUE
(common positive) wire of the decoder, and connect each -ve wire of the lighting board directly to either WHITE (forward motion) or YELLOW
(reverse motion) wires of the decoder. That's it!
Wrap your resistors in heat-shrink tubing, or electrical tape, fix into place with some tape and reassemble the loco.
If you are feeling adventurous, or space is so tight that you don't think you'll be be able to squeeze in standard sized resistors, then
you can consider chopping the original factory circuit board up, and re-using the tiny surface-mount resistors instead!