This is how the 2000 VW Beetle is, the 2004 should be similar:
There are round access panels more or less behind each tail light in the trunk. There is a small slot at the edge that you can insert a wide blade screwdriver to pop the panel out.
Once the panel is out, find the star shaped knob and unscrew (counterclockwise) to remove.
Now it will be possible to remove the entire tail light by using a thin flat blade. A putty knife, especially a plastic one would be good. Being careful not to damage the paint, slip the blade under the lower part of the tail light lens and pry the tail light assembly out.
Once the assembly is out, both the turn signal and stop/rear lamp sockets are exposed. The connector can be removed by pushing in a little tab on the side and pulling the connector straight out. It may not be absolutely necessary to remove the connector if the wiring is long enough, but when the connector is removed, it is easier to remove the lamp socket.
Remove the lamp socket by twisting slightly counterclockwise (only about 1/8 turn).
Remove the lamp by twisting slightly counterclockwise.
Note that the lamp base is a "bayonet" type with two pins sticking out the sides. One pin is closer to the base than the other one, so you must make sure to look inside the socket to make sure which pin goes to which side.
Assembly is reverse of above, but make sure when replacing tail light assembly to line up the upper tab of the assembly where it belongs in the hole first, then push down on bottom part of assembly to snap into place. Also pay attention to align the tabs of the round access panels when replacing.