CSF,
I never felt any layer of foam under the headliner cloth, but regardless, it sounds like its time to strip it out, and see what I have to work with. I have done some minor upholstery work, so if worse comes to worse, I may just use a similar color, foam backed vinyl, and do a button tuck style headliner. This is if the cardboard type backing is strong enough to take the additional weight, of the vinyl material and foam...

First step is to get it out! Are headliners secured, other then the dome light and the plastic trim pieces around the front and sides? There is no retainer clips in the roof itself is there? If there is I imagine they may be a little brittle after 25 years....