Plug is in the offside rear quarter behind the panel that has the little cubby in it (well, on the 3dr anyway) just behind the wheel arch. To get it off you need to remove the strip that runs the width of the car beneath the rear door, or at least the end couple of screws. There are 4 screws/clips which look like netting tie points that turn to release the panel then it peels out from the door/roof seals. Bugger to get back in but necessary. The hole for bringing the wires into the back of the car is also exposed when you remove this panel, on the boot floor in the corner.
Could scan in the instructions at work on monday of you're really struggling, but it's fairly straight forward.
Not sure on the relay question. I just did a straight swap between the kit (I think 4 pin) and original one.