I will run a separate fee line from the fuse box, hence a relay and separate dedicated fuse. Just me, I know there is not need but you can never be too safe in a electric circuit, extra layer of safety that's all but in reality a 20 amp fuse will be plenty protection for a low draw circuit like this

 

Paul in the UK sells 2 connectors that fit the 2 unused ports on the interior fuse box.

Here is his email.

lr_connectors@btinternet.com

 

most auto parts store sell these tap a line fuses
20A Regular Blade Fuse Tap Holder Add A Circuit Line ATO ATC Apr Car Truck | eBay

 

Here are pics of Paul's connectors. He also gives you the breakdown of what wires are what.

This is for S12:

The circuits are as follows -

Terminal-5 - Live - ignition switched (position 1 on ignition switch), via fuse F27 (10 Amps)

Terminal-4 - Live - permanent, via fuse F20 (15 Amps)

Terminal-3 - Live - switched, from auxiliary circuits relay, via fuse F15 (20 Amps)

Terminal-2 - Live - from main lighting switch (sidelights position), via fuse F33 (10 Amps)

Terminal-1 - Earth - via C0583 to C0017-1 (Earth Header) on 3.0mm² cable

 

I followed fishEH's install, but only put in the double usb port, since the stock one is real close. I also went with the connectors that abran posted about from Paul. The whole thing took 15 minutes. I also used the 15 amp pin, that is live at all times, since the stock one is switched.