Jealous of vpoppv's LED dome light upgrade, I had to do the same...
Well, I went and spent full retail price at the autoparts store for an LED Domelight replacement.
This one is a small PCB covered with an array of LEDs. It's roughly one inch by two inches.
The universal LED dome light came with three different "mock bulbs". Those are adapters that you pop in where your incandecent light would have been to supply power for the LED board.
The the LED board just gets double-sided taped in place.
A Geo Metro dome light is a simple thing. The switch for the light is built right into it. There isn't a lot of spare room under the cover, so it was a little bit of a trick to fit the LED board in there.
Here's the original heat bulb in place without the cover.
And here is the LED array (with the cover in place, which does act as a diffuser) with the camera on manual so it is at the same exposure as the other bulb photo.
You can see how much brighter the LED array is.
Anyone who is good with electronics could easily build a little circuit board of white LEDs and make one of these.
One feature I really like on a number of cars are the "footlights". When you open the door, there is a light down under the dashboard on both sides to see whats on the floor where your feet go. It might be fun to add lights under the dash on a Metro for better cabin light at night.