What is the best adhesive to tack the head liner back on. please help I cant see its driving me crazy?

Absolutely the only thing to use and the one that upholstery shops usually use is the 3M brand number 77 spray adhesive. Most all major auto parts stores carry it and if I remember is about 7 bucks a can. DON'T get the cheap stuff as is crappola. Heat will cause the cheap stuff to let go on a hot day. That 3M 77 stuff lasts at least as long as the original if not longer. Sprays to both sides (headliner cloth and also headliner board) Once dry to the touch, depending on weather from a couple of minutes to 30 minutes, "carefully" put it back on. You won't get a second chance usually as this stuff stick s on contact when you are putting it back on so some help is usually a good idea to make sure you don't stick it back in the wron and have it stuck there permanently. :-)