Before i get all questions asking why etc.

In UK you must have rear fog light. I dont want to cut one into mazdaspeed bumper. So i want to wire it into rear lights. question is how

 

If you want it to be always on with the taillights, you can easily hook it up to the license plate light wires. There is a grommet on the bottom of the trunk. You have to poke the wires through that.
That's how I have my derwankel rotary lamp connected.