Replace it. Joann Fabric has the gray headliner material with foam backing. Gotta remove all the old off the headliner backing board first though. A brass wire brush works well but use a light touch. Once the backing board is clean, brush on Dap Contact Cement to the backing board only, let it get tacky for a few minutes, then lay the new fabric on starting on one side. A 2-liter Coke bottle makes a good rolling pin to press the fabric down evenly lengthwise from side-to-side. A little patience goes a long way here. Work outdoors; that contact cement has powerful, flammable fumes. You can get dizzy pretty easy from it.