During this winter, one day I discovered Highbeam sign is on when I wasn't even turned on the headlamps. When I turned on the lights, I noticed that right headlamp is very dimm and left lamp is burning full force. Checked all the fuses, nothing wrong. Opened the headlamp assembly and found that light circuit (plastic adaptor with three wires) is short. Got a new one from Canadian tire, it worked for a few days and then the same problem happened again, this time with the other side. So far, I have replaced that plastic circuit three times and the problem appeared again last night...
Any clue why it's happening?
Thanks

 

Are you getting water or road spray on the wiring? Check your fender liner for cracks or holes.

 

There are no cracks or crevices water can get to that area, also on my second attempt, I had sealed those sockets with plastic glue gun. Also I noticed IT IS ALWAYS THE MIDDLE PORT which burns out and I know in 9007 type bulbs, middle is ground wire...

 

This makes no sense. The filaments of the bulbs are turned on by applying 12 volts to them through the wiring. The maximum current is limited by the resistance of the filaments of the bulbs. There's no additional voltage available, and any additional resistance would reduce the current, not increase it. If you had a low resistance short drawing current off somewhere, the bulbs probably wouldn't light at all, but the burned wiring should be in the feed path, not the ground path, since presumably, the bulbs would be bypassed to ground.
You might want to install an ammeter in that circuit to see what the current draw is. Get a reading on the bulbs for resistance to get an idea of what the current should be. If the current actually flowing is a lot more that what you expect, then there's a short somewhere. If the current actually flowing is about what you expect, then the plugs are not rated for the capacity. Try a different brand.