It's absolutely possible and heaps of people have done it. It's no more difficult or complicated than a "normal" car, except there are a few minor differences. Search these forums and you'll find tons of info. I used Travis Llado's blog as a main guide. Link below.
The main things you need to know:
- Take 12v from the penthouse under the rear seat. This is good for like 2000+W or something, and will prevent 12v battery errors. Read up more about this first if you're unfamiliar.
- Last I checked, no one has managed to decode the digital audio signals, so you need to take direct analogue speaker level signals, and feed them into your aftermarket amp.
I personally replaced all of my speakers, including a new sealed enclosure and sub, plus added an amp for the new sub and front door speakers.
Blog about various hobby and engineering projects including mechanical, electrical, and software
www.travisllado.com