British Rear fog light

Hello all, I am clueless when it comes to electronics although I'm willing to learn.

Basically, my car is an import from Japan, and doesn't have a rear fog light (MK2 MR2). I plan on adding an aftermarket one by either changing one of the reversing lights or by using the inner two lights. I also plan on putting a toggle switch with a separate LED. I would be most grateful if I could be given some advice please. And I also have some questions:

1. Why are there two sets of 3 terminals on the toggle switch, and why are the middle ones longer?

2. Will I have to (a) draw juice from the battery direct or can I (b) split the wire coming from the fuse box to the rear lights, add my switch and LED to it, then connect it to the bulb(s)?

3. There is a relay for the rear lights, and I have no idea what it is and what kind it is, and if I can do 2 (b) above, do I have to split it before or after the relay?

Many thanks,

Ash