Courtesy of an r32oc member
You will need some Thin Wall 1.0mm² 32/0.20mm loom wire. I ordered 10 meters of each colour so i had plenty. You will get away with thinner wire, however seeing as you are running brake AND tail, thicker is always better/safer.
Now, there are a few ways to do this. You could probably get away with running a single wire to the tail lights, and literally just use a ring connector to bolt the earth to the tailgate somewhere. However I did not go this route. In fact I ended up running 6 cables in total, but you will not need that many.
have a plan in your head on what you want to achieve and go from there. I ran 3 wires to each inner and splced them into BOTH rear lights. That way if I blow a fuse etc I will only have 2 lamps go down rather than 3 out of the 4. Hope that makes sense.
Anyway, if you go for the single wire way, connect your earth wire to a good earth point on the tailgate, ideally one for each inner. Then connect the feed wire to both inners and run it down the passenger side rubber boot on the tailgate/body. Pick up the wire the other side and feed it down the C pillar. At this point it's easier to remove the trim down the C pillar and boot. Feed the wire down paralell to existing wiring until you get to the tail light.
Here's the tricky to describe part. Depending on what effect you want, you will need to do some pin swapping. For the quad light (skyline) effect, remove the black/red (black/purple on the drivers side) pin from it's connector and splice your wire to the inners to this. Now remove the thinner wire (not sure on colours) in pin 3 position and put pin 2 in it's place. You should now have side lights and brake lights on your inners.
The wire that is still out of the connector is your tail light wire and you have a couple of options. 1. Tape it back (will bring the bulb out error on the dash). 2. fit a 6 ohm resistor in place (what i have done, will remove bulb out error). 3. Connect it to pin 2 position (will light the centre of the outer lights up when lights are on).
Now, depending on how OE you want this to look, you can run more wires (2 from each inner) the same way and connect them to each tail light. Or you can leave it as a single wire feeding both inners.The brown wire is earth, so you can splice into that if required.
As my SMD conversion uses indicators i ended up running 6 wires through the tailgate (3 wires per inner) and picking up earths at each outer light. Also, doing it this way should not require any VCDS programming, although you can have a play with the brightnesses etc if you want.
Hope that helps.
Kris