I'm not an electrician so don't take my word as gospel but this is how I did Lee_H's.
Get the two red wires and twist them both together, then do the same to the two black wires.
You then need to locate your sidelight wires which is easy enough. Find the positive wire, cut it, expose the wires and solder into a sort of T shape making sure the old circuit is back connected but the red wires are also attached.
Do the same for the black side and it should work.
Make sure you use plenty of electric tape to cover up all exposed wires.
Here is a crap diagram to try and make some sense of it for you.
I have attached a basic diagram which hopefully may explain things more clearly.
The wiring colour on the sidelight plug side isn't as shown in the diagram, I think it's brown and something else. I put it as red and black just to show how the wiring works.
use all 4 wires i had this problem and blew countless fuses red/black to one existing side light wire and red/black to the other try this it should work . they come out of two holes in the plastic but it doesn't mean they are differant (i dont think)