If you want to include the fog light & only run 2 core cable, it is best to bring the trailer cable into the OSR (driver's side)(there is a handy drain hole where two bits of bodywork join) .
I brought mine into the NSR (passenger) & so had to run the three core cable.
For those who are interested (& to save anyone else from pulling the light cluster & possibly losing the silly nut thingy), the wiring colours are...

In order on the plug.

Offside (driver)
Grey (or a strange purple - it was hard to tell) - Fog
Green/Yellow - Brake
Pink - Indicator
White - Reverse
Yellow/Red - Tail
Black - Earth

Nearside (passenger)
Unused Pin (for fog if wired)
Green/White - Brake
Green/Red - Indicator
White - Reverse
Yellow/Black - Tail
Black - Earth

I ran my wire up and over, through the plastic headlining above the tailgate.
All the wires were scotched in with no ill effects to the system. I took a 12v supply for the relay from the power socket by the gearstick. Again, scotched a wire to the supply for this, running under the centre console, under the carpet & through to the back behind the panel.
A good diagram for wiring the socket can be found at http://www.towbex.co.uk/cdata/3017/docs/601236_1.pdf