Have you ever ran aftermarket bulbs that were over the OEM recommended wattage? If so, you might have damaged the plug. I had to solder new connectors on both of mine because it basically melted them from higher wattage bulbs. if the connectors look ok use a multimeter and check the voltage at each headlight. If they both show virtually the same voltage, then it's likely the bulb. Have you tried swapping the bulbs with each other? You could also run a temporary ground to the driver side plug to see if that fixes the issue. Keep in mind when if you do that, your DRL will run at full brightness because of how the car controls the DRL.
In a case like this, you have to just start some trial-and-error elimination until you find the problem.
2003 Matrix Xrs Automatic
2011 Rav4 Sport