Ok, well I don't have the best camera in the world, and is usually very bad in low situations, but this gives you an idea of the light spread, out of one of these things...and for my camera to pick it up, it's bight, i can assue you.
In my setup, i haven't mounted it in the original interior light spot, because mine has been removed due to the fitting of a sunroof. But there is no reason why this wouldn't be able to be done.
This here is what I have used, the over all size of it is about the size of a 10c or $1 coin. There are 4 versions in White...a 1watt White, 1watt Warm White, 3watt White and 3watt Warm white. Warm just means its not as "white". The "white" ones output light like a fluro. I used the 3watt white.
Here is where I got the power supply circuit diagram from. You can buy specific little power supplies for them, but at a cost of $30 compared to $6, i built my own. However, the transistor/FET listed to be used on this site aren't available easily, but there are substitutes that I chased up. Again, anyone really interested in going this way, i can get you the part numbers of the parts i used.
http://www.instructables.com/id/Circuit ... wer-LED_s/