I
just ran a power lead off the battery for a small powered sub I'm mounting under the driver's side front seat! There is an existing rubber grommet and wires running through firewall, below the battery, towards the center of the vehicle, that pokes through on the inside behind the heater-A/C unit under the dash. Push a screw driver tip next to the existing OEM wire, and push or fish your wire through. If you need more info or a pic of the location I can provide one tomorrow.
Your sub amp should work just fine under the seat. It'll run cooler (out of the sun), won't be an eyesore (unless you LIKE exposed components), be less attractive to thieves, or take up valuable room from the rear compartment. If you use the heavy gauge speaker wire, you'll be fine.
Don't forget to "in-line" fuse your new 12V lead right at the battery terminal, and again use a large enough gauge wire to support the amp's power requirements.