Take the lens off, make sure it's clean inside the housing so it will reflect the light. Make sure the lens is not painted inside to dim the light.
Remove the bulb, what number is it? Jump power and ground to it. Does it get any brighter or stay dim?
Check for battery voltage at socket.
Check resistance to ground at bulb contact. The ground is switched by the jamb switches so it's on a terminal at bottom of socket, it is not a chassis ground on base of bulb.
