Could be a bad bulb tray, I was blowing bulbs weekly and i bought a new tray from the dealer and have not blown one since. Maybe swap the trays from one side to the other and see if it follows the tray? I have a weaker light output on my passenger side, never dug into it though so let me know what yo ufind out please.
When i get home i can look at the wiring on that tailight/ground specifically and see how it is wired. it may be something as simple as tightening a ground.
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