I've done at least eight headliner replacements. Wagons are easy since the headliner board will come out the tailgate. It takes two people to remove and replace the headliner board from a wagon. You must handle the board carefully to keep from breaking it. Once it is out turn it upside down and carefully remove all of the decayed material. I use a wire bristle brush to get rid of the foam residue. I use 3M Super 77 spray contact adhesive to glue the new headliner material. You'll need two people for this, to. To position the new headliner material in place you may want to use thick paper between the new headliner material and the headliner board. You could use cardboard. You slip the cardboard out and stick the new material to the board as you go. Don't expect perfect results on your first job.
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john