Okey dokey - the pics have arrived!
I started by putting the sub in the boot, on the side that i wanted it (I chose the single folding-down seat, which I never actually fold down, the double gets used a lot for my doggy

)
After positioning the sub where I wanted it, I went about planning how to actually get power to the back... hopefully I'll be able to save you the same confusion! Undo the glovebox, and you'll see what you can see below, except there was a little plastic cover bit over the first torx screw I targeted! I pinged it out, and nearly took my eye out, so go easy guys and gals!
If you're wondering what a torx screw is, its modelled in the sexiest way ever below. I think they were t20's. And yes, I do bite my nails
Next there are 3 (only 2 pictured!) more torx screws at the top of the glovebox compartment. I removed these, but as luck would have it, the glovebox still didn't budge! Doh!
Right, so to fully remove the glovebox, you should have already removed the 3 torx screws (in the 3 red circles along the top right of the image) Now there are only 2 more. To get to these, you have to firstly lift up the door sill (green circle!) and then you can remove the big "hockey stick bit" (yellow circle!) and tehn remove the end "cap" on teh glovebox (orange circle!). Now you should be able to get at the last 2 screws... and GERANIMO! the glove box should be all yours!
I forgot to take a pic of exactly where the grommet thingy is, but you if you can't see it, you might wanna put your car together and drive to somebody and pay them to do this for you! It's a whacking great rubber doo-hicky that requires a fair amount of force to pull it out! Go easy, as I don't want to be responsible for anybody screwing up their door looms! After faffing for ages with a laser pen to try and spot the corresponding hole on the engine side, I resorted to a vaguely straightened coat hanger. If you get right in to yoru footwell and peer through the grommet, you can just about see some daylight - that's your target. So get poking. I found that just pointing it in the right general direction and pushing it tentatively through bit by bit (whilst running around to the engine side of things frequently to check I wasn't ballsing anything up!) did the trick.
Anyway, on the outside, this is where it emerges (again, I nearly forgot to capture this, so excuse the blurry bit of my hook!) this is between the accessory cover (which I had removed the lid of to make more room (2 t30 torx's, if you need to take it out of the equation!)
A better image of my fashioned hook which i was able to thread the loop on the power cable over. Run back round to yoru passenger footwell, and give this a tug, and slowly but surely it should come out inside the car! Yay, getting close!
Back inside the car, this is the power lead all the way through, complete with my beloved coat hanger still attached!
Pull through all the slack, and you can begin laying this down along the sills. I hid the cable under the middle bit of trim (the yellow circled one in the pic above!) and let it poke through along the sill - as i found it a bit of a mission to get the sills to lay flat with the cable under!)
Carry on working your way down the car (bear in mind mine's a 5-door, so proabbly easier than 3-doors

) then on this (below) piece of trim, I fed the lead under it, and pulled it pretty tightly in to place (giving lots of spare in the boot, but reducing the trip hazard bits in front of the doors!)
Feed the cable up, and in to the boot to your amp (which in my case, is part of the sub!)
That's all for now, next chapter is the how-to on the remote lead (which means the sub won't be drawing power when there's no need) and the earth and RCA plugs, but they'll wait for when my head unit is here to plug it all in to!
Thanks for reading guys, hope it's a help so far!