There are plastic tabs that lock into a bracket underneath the headliner. My front tabs became brittle over time on my 03 eventually snapped, so it was hanging up there by the back tabs while the front was sagging down. You can pull the overhead by gently pulling down to get the tabs out of the mount bracket so you can see what's going on.
Now here's what sucked in my experience....the plastic housing the overhead and the lights are in are replaceable, but the newer part on the shelves only worked with an updated bracket that my truck was not assembled with. Bracket was like $150+ IIRC. Never replaced it, just extended my sun visors all the way out to support the overheadarty018:
Maybe the 04 already has updated housing and bracket, you'd have to talk to the parts counter.
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