Remove the license plate. Then remove the backup lamp assembly for that side.
Once that is off, you can reach in and remove the two 10mm nuts that hold the inside tail lamp assembly in place. Small hands and arms help here but expect to leave some skin :ugh If the bulb in the outside lamp is the one out, go thru the hole where the inside lamp sits.
If the bulbs are original, you will find that the sockets have some sort of waterproof dielectric grease that the factory added. Chances are that it has hardened to the point where it acts more like glue and the bulb glass will most likely break off from the base. If they do, use some needle-nose pliers to remove the base from the socket.
This is as good a time as any to replace all four bulbs (don't worry, the scrapes on both arms will heal in a few days :L ). Get new 1157 bulbs (Sylvania has some "Long Life" bulbs-1157LL) and pull the old bulbs, clean out the old grease/glue and clean the socket up, making sure that you scrape any gunk off the contacts in the base.
When you get the bulbs, get one of the small tubes of Permatex Dielectric grease and coat the bulb socket and the two contacts and re-insert into the sockets. Test the lights before you replace the lamp housings!!
Even though the lamp housings are supposed to be "sealed" you will find dust and dirt inside them. Pour in a few drops of dish soap and some hot water and shake the housing for a minute or so and then rinse out completely. Let them dry, and they will look brighter. A small amount of chrome polish on the lens will brighten up any dull spots.
