You can get some All Purpose Cleaner (it's a meguiars professional line product, but I think any generic one might work)spray it on a terry cloth towel and rub the towel into the headliner, and have a bottle of water with another terry cloth towel to follow. your goal is to clean it, not get it soaked. give it a try in a small area first.
it worked on every other vw mk4 headliner i've done. good luck
it worked on every other vw mk4 headliner i've done. good luck