One dim parking light?

Take a peice of wire, ground one end to the battery negative and touch the other end to the bulb base while it's in the socket and turned on. (The bulb base, and not the socket metal)
If it gets brighter then you know there is a ground issue.
If it doesn't, then you know it has low voltage.

On some of these old lights I have found that the metal housing the bulb socket is crimped into has a crappy ground to the socket. (I guess corrosion gets between the metals)