zedonist said:
Its probably a bad earth (connection) causing higher resistance, it is drawing more current than it needs, working hotter and burning the filament out, a
Here comes the leckie geeks point of view. More resistance will lower the current (think of current as electrical flow, the more you resist it the less you get). The two brake bulbs are wired in parallel, it stands to reason that the same one keeps blowing so maybe the side that isn't blowing could be high resistance. This would cause more current to flow through the side that is blowing. Basicaly, replace both bulbs and clean up the contacts on both sides.
The level of current is set by the resistance and voltage. If the bulb thing doesn't work have your alternator checked out, it might be regulating your voltage slighty too high, more voltage= more current. Also, make sure you have the correct rating of fuse fitted for that circuit.
Only other thing I can think of is a possible intermitant short to earth. This could be anywhere in the electrical system and would be extremely hard to find.
I like the shovel idea though. :-bd