Have you checked the connections on the bulb and the plug for corrosion? Sounds like high resistance... Next thing to do would be to check for 12 volts at the plug. If you get 12 volts its likely to be a dodgy bulb, if you get less then keep working backwards until you find the source of your problem...
Just check the bulb mate...it sounds like it is justing showing as the park light...and not the proper main beam (not to be confused with the high beam!). I reckon the filament has gone.
Flick the light switch one position to parking light, and have a look at the brightness, then up the switch to main beam and see it it increases in brightness in any way.
If not, then I think its just the filament for the main beam gone. If it does a little, but not enough, then perhaps the problem lies elsewhere as mentioned before.