Having spent the best part of an hour trying to run a cable through the bulk head, I have given up.
I will re run the power or my light bar from the rear of the car.

I looked for this mythical grommet, but think it is as real as fairy dust on a MY13
Not really happy with using the bonnet release method - as I dont want remove the battery and tray, and then
subsequently reset my windows roof (radio ?) etc

So, through the boot it is then.

Will make a start this weekend, and remove the roof rails completly to hid the cable in the gully.
Its a shame really, I only need to run the +12v, I have tested using the roof rail mount as an earth and it is fine

EDIT

Now sorted - thanks to tip from Dougbay

The flap Dougbay mentions is totally out of sight.
It took me less than 10 minutes to remove glove box and pass a cable through

Just need to identify what the flap actually does, and will having a cable through there
cause any issues.

Below is a picture of where it comes out behind the glove box.

the shiny black wire running up / down right in the middle is my cable.

Click image to enlarge

Are you after a live feed from within the engine compartment? 2013 Dynamic Santorini black
2011 XS Lago grey
2007 HLS Stornoway grey
sometimes towing an Elldis AVANTE 574.

Plenty of live feeds available in the fuse box.
It's the cable run via the bulk head for the switch that's
The issue. 

When I wired the override switch for the driving lights on my 2007 model I removed the glove box and routed the switch to relay cable, through the bonnet release cable grommet. Absolute pain in the neck but managed it eventually after an hour. Be careful because it’s easy to push the grommet out into the engine compartment and is a job and a half to fit it back in place.
Best of luck.  Gone:
2002 FL1 TD4 GS Zambezi Silver
2007 FL2 TD4 GS Izmir Blue

Should have added to my last post, I used ‘Thinwall’ cable to make it easier. Gone:
2002 FL1 TD4 GS Zambezi Silver
2007 FL2 TD4 GS Izmir Blue

I am a radio amateur and needed a 12 v supply for my radio that can draw 20 amps.Like you I searched for over an hour for somewhere to run a cable from the battery to the interior.Found a perfect solution.
Remove battery cover. Feel behind battery towards the centre line of the car. You will feel a large rubber flap hidden from view probably there to equalise air pressure in the cabin when a door is shut. Push cable through it and it and the cable should appear in the front passenger footwell. Job done.

That's a great tip dougbay
I'am in to cb radios and what to go direct to battery trying to stop cooling fan interference

No flap on mine
Perhaps it's a model year thing 

My one with the flap is a 2013.

Do you have a picture pls. 

I will try and put a picture up tomorrow if I can work out how to do it.
There is a sort of double bulkhead on the FL2, by removing the battery cover you can get your hand in between the two bulkheads. If you carefully feel along the rear bulkhead you should be able to locate the rubber flap.It is sort of hinged and about 4 inches diameter.

I have uploaded 3 photos to my gallery (I think) to show the cable run from the battery to the interior.If you cannot see them let me know and I will try again.Don't forget to put a fuse in the +ve line near the battery.

thanks I can see them

This looks like the cables are running into the chamber for the heater blower.
I have a rubber plug in there, but it is fixed.

Will remove the "Plenum Chamber cover" plastic covers that covers the entire bulkhead and have a look further later 

The cable is not going into the heater but directly into the cabin.

superstar.

looks like this may be the way forward.
The flap you mention is totally out of sight.
It took me less than 10 minutes to remove glove box and pass a cable through

Just need to identify what the flap actually does, and will having a cable through there
cause any issues.

Below is a picture of where it comes out behind the glove box.

the shiny black wire running up / down right in the middle is my cable.

Click image to enlarge