Disconnect the negative (-) battery terminal.
Open the trunk lid and then disconnect the connector of high mounted stop lamp.
Remove the package tray.
Replace the bulb from the package tray.
Installation is the reverse of removal.
I fought with changing the brake light per the service manual on the Hyundai site. (The manual requires removing trim in the back seat and the "package-tray" in the back window to gain access!) After about an hour, I decided that there had to be a better way. There is no way that a dealer is going to remove all the trim in the back window to change a light bulb. I was determined to find their shortcut.
With the package tray pealed back as best I could, I still could not adequately maneuver the light socket let alone change out a bulb and replace it. I could, however, see the light socket enough to be able to reach into the trunk through the rear seat passenger compartment and marked the spot under the bulb. I then cut my own access hole with a 2-1/4 inch hole-saw through both layers of sheet metal of the trunk "ceiling" directly below the light.
On the 2007 Sonata trunk ceiling (not the lid!) there are raised portions of sheet metal, directly below the light there is one of these raised portions. I cut my hole just aft of the center of the raised portion which just so happened to be about 2-1/4 inches across.
My saw cut though both layers at about 3/4" deep and my pilot bit set maybe an 1/8" further out. I ensured that all wires were clear using a nearby hole. Remember to de-bur your holes before getting your fingers in there!!!
From now on my light bulb changes will be a relatively straight forward procedure. Hope this works for someone else out there! I have posted a photo that may help. Good luck! http://i818.photobucket.com/albums/zz107/Lindberg76/third%20brake%20light%202007%20Hyundai%20Sonata/IMG_3052anotated.jpg