The headliner in my van has gotten horribly dirty and I need to clean it. Actually it was kind of dirty when I bought the van but it has gotten worse after my kids have done their job. So should I take the headliner out to clean it or just try to clean it while it's still up and attached? I would think it would be easier to clean if I got it out and laid it on a flat surface, but I'm open to ideals here.