Well, there are a few facts...
If its a "Mercury bulb", (white light), they tend to lose thier efficiency very quickly over the years. As the Mercury in the glass tends to leak, or "gas out" quickly...
Sodium lights will last longer, but doesnt put out the white light, but an orange light, but will last a long time.
Any type of bulb, being hot, should not have cold water splashed on it. Even though the bulbs are in a "vacumn", the glass will crack after repeated cold spots on a hot glass.
I have seen regular bulbs used in glass enclosures. Or, if they have a large hood over them to protect them from the elements, (by hood, meaning a 4" hood all around, 8" diameter).
If you know where the "fuse" or "breaker" is for the light, then you can get a signal generator, (probably about 25 bucks or so), and unhook the wire, (after shutting the power off), and hooking up the signal generator to the wire and you take the wand and go along the ground and you can actually follow the line! I have one and love it! (I even use it to follor sewer lines, after putting down a metal snake, and attaching the clips from the signal generator to that!)...
Now, according to my Professor from Ohio Universtiy, the electrical current is on the "outside" of the wiring. So, if you get exposed copper, it turns green, and thus stops the flow of electrons because they cant travel in the green stuff, thus making an efficient barrier. Or, the flow is sufficiently reduced to stop it from lighting the light. Test with a volt-ohm meter. If its broke, due to expnsion and contraction of the soil, because its not burried deep enough and has been damaged by other things, as a piece of machinery too heavy for the wire to bear under a small amount of soil, then it will have been seperated (probably the copper and the wire sheath which will stretch can still be intact), and thus not working also.
Just some things to toss around and think about. Wires should be burried about 3 feet deep, (two feet very minimum), and marked. They suggest to leave a "barrier" of signs one to two feet down, before you hit the lines, as a warning. (This barrier is just a tape with the words underground lines along it)...
Living next to an electrical contractor store, (which I frequent, because of the friends I have made there and as a customer), I get to talk to contractors, and learn a lot from them and the store employess.
Just my two cents for what its worth, and a wee bit extra for the collection plate...
Jesse