+1 on the door panels being super easy. I had a jeep cherokee that I absolutely hated having to get into the doors, because of the ~15 one-time-use pop rivets. Taking off the frontier doors were so much easier-- 4 screws, a few friction-fit clips, then it comes right off.
Here's how I did my speaker upgrades: http://www.clubfrontier.org/forums/f102/red-baron-133841/index2.html#post2658993
Here's how I did my speaker upgrades: http://www.clubfrontier.org/forums/f102/red-baron-133841/index2.html#post2658993