I have 6 x light bar ready Dapol carriages and a self adhesive strip of 12V LEDs which can be used in short sections. I wondered if I could solder the positive and negative of the LED strips to the correct plugs which I think are available on the internet and use them in the existing LED light bar ready socket in the carriages. My query is that the socket seems to be for 3 wires & not just a positive and negative feed. Can anyone comment as to what the extra hole is for please? Many thanks. 

Have a look at my tutorial for fitting fibre optic tail lamps to light bar ready coaches as it covers and shows the fitting of the cheap led strip light bars and the type of connector and wiring used.  Basically the middle black wire of the mating plug is not used and can be cut off.  See http://www.ngaugeforum.co.uk/SMFN/index.php?topic=4022.msg45608#msg45608

Hi Caz,
Many thanks for the info and very useful tutorial.
Have just order the connectors from the seller you advise.
Cheers,