Hi!
I am planning to do the same (along with the front speakers), and there is a pretty comprehensive guide in the guides section titled "Changing the rear speakers (Oh My!)" (sorry can't post links, just use the search).
It's not easy, it requires you to blast open the entire back of the interior, take out the seats and peel back the interior cover, so prepare for a grueling 3 hour work. But I hear it's actually worth it