Solution: Push the entire unit (not just clear plastic cover) from the driver side toward the passenger side, then pull down.
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I was unable to remove the dome light cover in my 1998 Saturn SL.
I searched through Google groups and came across a couple of other posts on the subject with no solution. I called one Saturn service center where they told me all you have to do is squeeze and pull. I called another and they said a "crochet-hook"-like tool is needed to remove it, by inserting it into the notch on the passenger side.
I don't have such a tool, so I ended up breaking it. But now I think I know the solution.
First, you cannot remove the clear plastic cover, as far as I can tell. Don't bother with the small notch on the passenger side of the cover. Trying to wedge that is how I broke mine.
It appears that all you have to do is push the entire unit from the driver side, then pull down. The light bulb can be easily replaced from the other side.
There are three hooks. Two on the passenger side, one larger one on the driver side. The passenger side hooks do not allow movement.
So, I got the cover off. The bulb looks fine. While moving it around to check the circuit with my multi-meter, the light came on, but went off again. There was still voltage, so it may still be a bad bulb. I haven't gotten a new one yet.