The header panel, or reveal moulding as Mercedes calls it, is not that hard to remove. The first step is the hardest.

1. Remove the A-pillar trim by prying between the bottom edge and the A-pillar, then at the top. Push the bottom forward and the top back and work the trim out. Mercedes recommends a flat plastic wedge. You may have to pry pretty hard to get it unclipped as the clips really grip to hold. This is the step that is the most nerve racking for fear of damaging the trim.
2. Remove the rear view mirror by yanking it backwards, grabbing right behind the mirror head (but not the head). The mirror is spring-loaded. Then remove the three screws holding the base plate.
3. Pry out (carefully) the center light with a small screwdriver (there is a notch to put the blade in). Unplug.
4. Remove the sunvisors by removing the two screws at the pivot point for each. Remove the center plastic clip by opening the screw flap with a small screwdriver and taking the single screw out.
5. If you have a sunroof, remove the long chrome rail at the rear of the panel and front of the sunroof opening.
6. Pull the panel out of the clips. It should slide out easily. Be careful not to rip the clips off of the panel, but if you are replacing it, I guess that is not a concern.