That dome light shouldn't be too hard to retrofit into current HHRs. I bet you just have to enlarge the hole in the headliner and it'll pop right in.

 

Speaking of which, am I the only one who thinks the headliner is really flimsy the way it is attached? I have the sunroof, and if I pull down at all on the headliner is feels like it will come right off the ceiling. I know....I know....don't pull down on the headliner....