If you only require a small power wire (8 gauge) for a small amp then you can use the grommet for the rear washer pipe behind the battery.
for larger wires (4 gauge +) you can drill extra holes and use Gland nuts to seal/secure it in the bulk head.
I've used 4 gauge for a 500 watt JL amp in the black Tigra (no rear washer) and 2 runs of 2 gauge for a 1800 watt 2 amp set up in the blue Corsa.
You can get to the inside car of this area by removing the glove box.
All depends on how much you want to spend too, I went overkill with a previous install, I still have a prototype sub in the garage I built for my old Corsa B out of a bit of sewer pipe lol.
I can't remember what gauge leads I used but it was bigger than what RING offered in Motorworld back in the day, I bought all the gear off ebay and build several installs with it.
If you don't want you lights to dim on each beat I recommend fitting a power cap, plus a decent amp to power the sup goes a long way, these Alpine units are awesome, cheap as chips in ebay if you can find one.