On my 2002, the heads of the fir tree clips slipped into slots in the door panel. After installing all the fir tree clips in the door panel, you then pushed the points of the fir tree clips into corresponding holes in the inner door skin to get them started and, once you had them all started, push the panel in flush against the inner door skin to set and lock the clips. I still have the flat, forked tool used to remove the door panels without breaking the clips (maybe!).
I never had the door panels off my 1996, so I don't know if they used the fir tree design or not. Even the metal spring clips used on Fords slipped into the door panels and then were inserted into holes in the inner door skin.
Rusty