You'll have to disassemble some of the interior to run the wires through the car.If only a small active subwoofer you can run the wire from the fusebox, instead of going "all the way" to the battery in front.
If the sub has hi-level in, you can connect it directly to the speaker-wires, without having to remove the radio/dash.
If you want an even better advice, then forget the sub. Install a 4-channel amplifier instead, and some new speakers in the car. This will give you a much better respond than having only a sub.
You can install the amplifier under the car seat, if using a smaller type (there's plenty of room under the seats for this).