Head light

If it actually has separate bulbs and circuits for high and low I would check the grounds first. But nothing I have shows that car as having two bulbs, high/low in one light shows for us/canada. In that case I would check the voltage going to the low beam side.

Precisely, and also the ground. Corrosion somewhere is likely at fault.

--scott

What the OP should be checking for is *voltage drops* through the circuit from supply to ground. That will indicate high resistances at any connector.

did you just recently change the bulb? You may have accidentally touched the bulb glass with your fingertips. Wear rubber gloves and remove the bulb and clean it with rubbing alcohol. It should be fine.

The oil from fingers will not directly harm the filament. What oil on the bulb glass does is change the heat characteristics of the bulb so that stresses are creates as the bulb heats up. These stresses will crack the glass and, once that happens, the filament is *history*, usually in less than a second.

For a filament to be less bright than it should, you need to be looking for undesirable circuit resistance which will cut down the current passing though the filament. This resistance can be isolated by carrying out voltage drop tests on the headlight circuits. The most common places for increased resistance in light circuits is at the earth return. Corrosion is the usual suspect.

I sell auto parts. I've seen this multiple times when people touch the glass with their bare hands.