I fitted mine in the boot and took a feed from the rear passenger speaker, splicing a cable in and running under the carpet. I removed my Airbox to drill a hole through the bulkhead and install a grommet for the positive feed. My Neutral feed was placed under the rear seat bolt.
FYI this was for fitting an infinity Basslink in the boot. Not particularly recommended as it's not a good fit, and I had to make a weird block to angle the sub so I could access the 'hidden boot' (therefore I could have probably made an enclosure to fit perfectly for similar cost and time...)