This is a little tricky.
They could have and should have made this much easier.


Look at the pictures in the first reply.

Do NOT remove the upper tray.
Do NOT remove the light housing attached to upper tray.
Do NOT brute force this no matter how tempting

You will need
1. a long bent needle nose pliers (this is critical)
- Hyundai line workers must have a special tool
2. phillips screw driver
3. flashlight (a long LED probe light would be good), and
4. patience

Lie in trunk
Disconnect the rear upper brake light power connection.
by pinching the bracket connection at bottom (white plastic)
The bracket is held in place with a silver phillips screw.
Unscrew the phillips screw.
Pull down the bracket.
The bracket wire is attached to the bulb holder still in the light housing
Look at the picture above.
Look at the "finned" bulb holder.
Those fins are what you will grab and twist.
You need to gently grasp it with the bent/curved needle nose.
A quarter turn will release it.
Pull it out and replace bulb.
Check light (reconnect) before reversing all this.

An access hole lined up with the finned bulb holder would make this much easier, but it can be done.

 

Or just reach up when in the trunk and remove the socket. It is hard to do but it is a twist lock assy. I shaved the locking mechanism so it will be easier next time. The biggest difficulty is the short wire on the socket just barely lets you rotate it down to pull the bulb out of it. But it can be done this way, lots easier.

 

I just finished replacing the bulb. I had to disconnect the wire harness plug then take a screwdriver to pry the bulb housing. By this I mean I had to rotate the housing using the frame as a fulcrum. The housing rotated and I was able to pry off the bulb assembly. At first I was confused by the chrome clip held on by a screw. I thought I had to remove this. You do not. Leave it be. I then removed the 2 small screws holding the light assembly. Be careful not to drop these screws. Use a magnetic screw bit to hold onto them. One of the screws is accessed through the rectangular hole in the trunk ceiling and the other is through a 3/4" circular hole on the other side. You will have to remove some tape that covers this hole. Go into the back of the car and now remove the light housing. Install the new bulb then put the bulb assembly into the light housing. Before installing the housing, cut the plastic tab that comes off the bulb assembly. It is designed to give you a place to grip the assembly and twist but you don't need it. You will not be able to replace the light housing unless you do this. You might have to press down a little to get it in but it should go. Again when replacing the screws be careful not to lose them (use magnetic bit as before). I had to use a fender washer on the screw that goes through the rectangular hole because it would not grip the upper deck. Get out of the trunk, have a beer and count the money you saved.

 

Good work, thanks for sharing, everybody has trouble with this.
Also thanks for spelling "lose" right, for some reason many people write "loose", and I am uncomfortable with telling them.