Depending on the model, you have either a cloth-like, vinyl-like or MB-Tex-type material as a headliner...you would use different products on each.
I would go with a leather cleaner on the MB-Tex, Simple Green on the vinyl, or Tuff Stuff on the cloth.
Each of my MBs have a different headliner material, and this is what works for me. My REAL problem isn't the sunroof area (although it does get dirty), but my wife's makeup...somehow, she gets it on the visors, pillars, doors, etc...
