One Of my headlights on the left, passenger side, of my 2005 honda civic lx manual 5 speed 4.1 vtech is significantly dimmer. Both the low beam and high beam are affected but not the turn indicators.
I bought two new headlamps because the old ones were so worn and thats when i noticed the issue, not that i could even tell with the old lamps. Driving does not effect the car so I do not think it is the alternator
I have performed the following trying to trouble shoot the issue:

Any help out there... baffled?

 

I would have said connector ground but if you already checked it thats good.
Getting a voltage of 12V+ is great but what does it get under load? ytou could be getting a voltage drop when under load due to bad grounds and/or corrosion even if you measure 12V no load

if you are not able to figure it out:

Step 1: Buy a $10 relay
Step 2: Connect the relay to run the lights off the battery directly

should take you about 20min to do this

hope this helps