So my headliner was sagging in the middle so I took it out and pealed back the fabric. All the foam stayed to the hard backing. I assumed the foam had wasted away to crumbles but it is actually in really good shape. Should I just re-glue the fabric to the foam, or remove all the foam and glue it to the hard back? I want to reuse the fabric since it is still in great shape.

Had mine replaced two months ago as it was sagging in a number of places.

This is not an easy process as it requires taking out the board from the ceiling, cleaning the foam and glue from it, regluing the foam and fabric and then reinstalling the board.

Mine was done in two hours at my house by a mobile installer for $250.

I doubt that it can be done DIY for much less than that as you will need to purchase new fabric.

The foam is attached to the fabric so if the fabric has separated from the foam it usually means you need to replace the whole headliner. Usually the foam dries out and separates from the fabric. An auto upholsterer shop or dealer can install the same color headliner. You can also remove the foam and buy the headliner material and install it.

You can use a high grade spray on upholstery adhesive if you've been able to peel the headliner all the way back. Let it get slightly tacky for a moment or two before you press the fabric onto the adhesive. If you wish to replace the fabric, in my 2010 XKR, I have the Alcantara suede headliner. A very similar synthetic suede fabric is available in over a 100 colors for well under $10 per running yard (60" width) in the textile district of any major city (or if you can't find it, let me know what color you need, and I'll be happy to pick it up and ship it) I pay about $5.00/yd wholesale. It is every bit the quality of the Alcantara brand and you should be able to do the whole things including "A" pillars in less than 3 yards.