I started another post about adding a new front dome light into the headliner and several people asked about how to remove the headliner.
Here are the steps for my 2008 without a Sunroof.
1. Remove the two interior A-piller covers. (Located on each side of the dashboard to the roof) Take out the screw behind a cover at the top of it, then pull it up and cover will come off. Then disconnect the tweeter connector.
2. Remove the two interior B-piller covers. (Located between the front and back doors) Unsnap the top seat belt bolt cover and remove the bolt holding the belt on. Then pull the cover toward you carefully unsnapping the plastic connectors. Now pull the cover up and out of the cover below it.
3. Remove the middle brake light cover by just pulling it unsnapping the connectors.
4. Remove the interior cover which surrounds the back window. Remove the back seat shoulder harnesses, one bolt each then just snap off the cover by carefully pulling on it.
5. Remove the 3 bolts holding on each of the Sun Visors and one bolt holding the visors end snap connector.
6. Remove the clothes hanger bracket located on the Headliner sides above the back seat. One screw each.
7. There is an electrical cable glued to the top side of the Headliner that connects to the Onstar equipped rear view mirror and dome lights that need to be disconnected. The mirror connector has a 6 or 8 pin connector that plugs into the back side of it. just pull it out here. The electrical feed in connector for these are located just below the drivers side rear corner. (just below the lift gate support shock.) It's pretty good sized, just disconnect it.
8. Remove all 4 of the seat headrest. Fold the back and passenger seats down and fully recline the drivers seat.
9. At this point the only thing holding the liner up is the rubber door seals around the 4 doors. Just gradually work the liner out of it on one side and that side of the liner will now lower. Do the same on the other side and down the liner will come.
10. Turn the Headliner at an angle. (one side up and the other down) Slightly bowing the liner slightly, gradually pull it out the back.
That's all there is to it. takes about an hour. If you have a Sun Roof there may be a few other steps. But I can't imagine there being much else to it.
Here is a picture of the liner removed. You may notice the added dome light.