The best thing to do if you are considering a sub, is to buy and wire up a suitable Amp to your boot, then run the speaker wire from your amp to your sub.
You will need to run some power wire from the battery, some RCA cables and a remote (thin power cable for headunit to tell amp when to switch on so it's not constantly live) from the headunit to the boot, and a ground to the amp (bare metal point near to the amp). The power wire must be fused within about 12 inches from the battery.
If you are unhappy with removing just the rear seat (2 bolts) then I'd suggest getting an amp installed professionally as it's a loads more hassle and work, especially in a coupe. Whats your budget for sub and amp?
Sideways a LOT
Re: wiring for a sub#184205 04/09/200615:5104/09/200615:51
I ran the RCA leads down the transmission tunnel as I prefer not to have it next to the power cable. Power cable and remote were run down the side next to the passenger door (there is already a lead that runs along there so just follow that).
I removed the rear seat (just the bottom - 2 bolts) then took out the passenger seat - 4 bolts. Removed the passenger door rubber which allowed the carpet to moved. Then removed the passenger seat belt bar at the botton of the rear door card (2 bolts) then prised off the rear door card at the bottom to gain access so that I could thread the power cables and remote. Then wriggled the carpet out from under the door card so that I had more access.
The above took about 2 hours to remove/refit.
From there I was able to thread the rca cable down transmission tunnel and run a new remote lead from front to back (I could have run the power lead as well, but there was already one in place).
You only need to remove the rear seat base to put in wires, so it's only 2 allen key bolts. A long piece of stiff wire should get you under the rest of the carpet.