it is not likely the wiring. it sounds like the one of the filaments (the thin wire looking thing in the bulb) is cut, usually the low beam since you have h4's. you look at the bulb and it will look alright (both ends of the filament), but one end is cut, so it may touch and get power and come on, but a vibration or impact can make it move and the light will not come on.

with the bulb out, hold it in your hand and flick it wth your other hand. like you were going to flick away a bug or something. then look at the filament. you will probably be able to see the filament vibrate and you'll be able to tell what end if cut. don;t worry about breaking it when you flick it either. you could throw it against the wall and i doubt it would break. ive tried.

to change, pull off the black thing, pull out the harness, and release the (wire) clip to get the bulb out.

since you have an EJ/EK, i wouldn't be suprised if the problem bulb was the driver side low beam... it's a common problem.