How to fit a subwoofer in citroen c1

Hi there, today i changed my stock speakers for some better ones which was very simple just 2 nuts and then just swap over. But i want to install an active subwoofer and will need to take the wiring from the boot to the battery. I need tips on where to run the wire through and if i use the nipple on the firewall which is a hole for fog lights if u add them where does it actually lead to inside the car. I can see it from inside the engine bay but im wondering if its behind the steering wheel to access it from inside the car? thanks