Problem fitting lights to hornby R decoder

[TrainDad]

I'm trying to fit lights to a class 47 loco fitted with a Hornby R decoder.  I've got the directional front and rear lights working correctly.    However, I want to have the front single headlight to work seperately to the directionally controlled twin marker light/tail combination.  Put simply I want the loco to have controllable directional marker lights and tail lights with the ability to control the single headlight (fitted to one end of the loco) independently.

I've tried connecting the single LED with resistor to the spare puple wire from the decoder and the blue common wire - as the instruction leaflet appears to indicate this would be correct and allow it to be controlled via function 3.  However, the LED comes on as soon as the train is on the track and cannot be controlled by any function key.  

To complicate matters - the loco is an old model and has been hard wired and a harness fitted to which the decoder is plugged.  The harness has a spare green wire.  I've tried conncting the single LED headlight and resistor to this and the common blue - the result being the LED comes on immediately the loco is powered but can be switched-off if function 3 is turned-on.

Has anyone any experience with this issue or advice?  Cheers.

[Chrissaf]

Does this light come on when not wired at all to the decoder. If it does, then that indicates that it is still wired in some way to pick up power from the track. The light needs to be totally isolated from any remnants of loco wiring / PCB circuit board (if fitted) before it can be controlled by the decoder.

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[TrainDad]

Thanks Chrissaf.  I've got it to work and are feeling a little stupid for not checking and wasting the forum's time..  I changed the polarity on the LED when conncted to the green wire from the harness and blue common which solved the problem.  The headlight can now be switched-on and off by using function 1.