I'm not sure if any of this is already common knowledge but I had trouble finding anything on the forum so hopefully it'll be helpful for some.
The 12v circuit I tee'd off was a non essential circuit for the rear seat heater switch and flashlight charger, it's fed from fuse F96 (10A).
To get to it, release the fuse box via the two holding clips at the front and back. If you have trouble with this, try going for a drive around the block, the heat from the engine will soften up the plastics alittle bit and make them more user friendly.
Once you've got the fuse box out you'll want the green plug. I believe all the pink/yellow wires are switched 12v supply wires but one you'll want, is the one in slot 15. If you have trouble finding 15 written on the back of the plug, its because they only show even numbers.