I recently had to change out one of the taillights in my 2005 Honda Civic Hybrid, and was surprised that it was more complex an operation than expected, requiring the partial removal of the bumper. It wasn't really difficult, but documentation was sparse, so I thought I'd share the experience here for anyone else encountering this situation without a friendly guide.
The steps laid out here aren't necessarily in the "correct" order, but this is the order in which I operated.
Step 1: Step One: in the Trunk
Expecting the best, I began in the trunk. The first thing to do is get in behind the trunk lining to access the back of the light. There are two little plastic plugs you can unscrew. Then just pull the lining clear of your work area. This part is described in the owner's manual, because this is also how you get in there to change the bulbs.
Once the liner is out of the way, it's pretty obvious what needs to be done. First, unplug the cables, then unscrew the nuts from the three posts holding the light lens in place.
At this point I started pulling on the lens, pushing on it, and was about to start prying, when I decided that there must be something more to this. I checked online, and discovered that I was going to have to remove the bumper to access the final screw. That's when I decided to begin recording the process.
Step 2: Step Two: Just Outside the Trunk
On the back of the bumper under where the trunk closes, and near the taillight, you'll notice a little black plug which you can pop out with a screwdriver or fingernail. Under that is a bolt you'll need a socket to remove. Do that.
Step 3: Step Three: in the Wheel-well
The "bumper" wraps around the back of the car up to the rear wheel-well. Inside the rim, coming up from the bottom, there are three screws (these will also be holding the "mudflap" in place if you have one) you'll need to remove. Above those is a little notch with a bolt you'll need a rachet to remove (you might be able to get it with a wrench).
Once the screws (and bolt) are out you might give the bumper a little tug to pop it free of the little semi-clip tabs in the lip under the main body of the car.
Step 4: Step Five: Under the Bumper
Under the bumper you'll find a phillips-head screw to unscrew, and a number of plastic "plugs" to pop out. These can be a little tricky, but not too bad. Just pry the central "push-in" part til it's sticking out about a quater to half an inch. Grab it (I like to use needlenose pliers) and pull it straigt out. It might take a little wiggling.
These "plugs" will sometimes break, so it's not bad to have some spares on-hand. These are available at autoparts stores. Look for 1/2 inch nylon rivets. I keep some in the trunk since they seem to be surprisingly integral to the structure of the car.
Step 5: Step Five: the Final Screw
Now that the bumper is hanging you can get in under the taillight bracket, unscrew that final screw, and pull out the lens. The bracket the taillight lens is attached to is kind of flexible, and I found it easier to bend it down a little to get the replacement lens in place, then bend it back up.
There you go. Hope it all works out for you!