Those little bulbs seem to have a short life. I've needed to replace both of them over the past couple of years.
Next time your close to a Auto Supply store, go in and buy a good brand of #53 bulbs.
Replacing them is easy. Just remove the phillips head screw and the metal retainer.
Then, pull the rubber sleeve out of the casquette and pry the lens out of it.
A 1/4 turn counterclockwise on the old bulb and it will come out.
When installing the new bulb make sure the two pins on the side of its base align with the grooves in the socket, push it in, turn it 1/4 turn clockwise, push the lens into the rubber and reinstall the metal retainer and it's fixed.
edit: (I hope you don't take offense when I seem to do an overkill on my descriptions of how to do something. I figure someday someone who is new to the world of mechanical things might read my comments so I gear them towards those people.
)