My 1982 Montecarlo headliner is sagging through out, do I need to replace the foam that is left or just glue replacment cloth to the foam, never did this before?

Headliner material is a combination of knitted cloth laminated to 3/16 inch or 1/8 inch foam, so the answer to the question would be; you would need to replace both seeing that headliner material comes as a cloth/foam combination. It is the foam deterioating that causes the headliner to sag and fall on your head.