This problem has been ongoing for years and few mechanics look at this and everyone seems have no clue.
My car is the 2011 model - whenever I use my car key to unlock the car (the big circle light at the back should lit up). Sometimes that circle lights on both sides both lits up, but for most times only 1 circle light lits up and stays on, the other one flash a few times and then goes off. When the car engine is on and I turn head lights on, the circle light which previously made the flashing would stay off as taillight. The other one is on as taillight with no problems. There are times the whole situation is reversed for the lights which is could be this time left is on as taillight, right hand side is gone. While some times the left is off, and right hand side is fine. It is just so weird. 3 weeks ago my garage charged me $400 AUD to replace the motherboard and unfortunately the problem still staying.
Few years ago I actually thought initially that was due to the entire backlight module so I replaced one of them entirely buying from a wrecker site for $100 - of coz the problems still exists unfortunately. Mitsubishi checked the car wiring for the backlights few years ago when it still has warranty they said wiring was fine. Recently I asked them again they said as the car now is out of warranty they have to charge $200 for inspection costs before they can do anything (this is totally BS and I basically won't take the car back to the dealership for any repairs as this appears to me as they are not trustworthy)
Some garage said the entire backlight modules has to be replaced (in which I don't believe this is the root of the problem) as the signal light at the back is fully functional, brake light is fine whenever I press the brake the circle light is on. It just seems that when it comes to tail-light the circle lights can't be both staying on. (as you know for this 2011 model the circle light acts as both taillight and brake light)
This still remain mystery and I hate to spend lots of money on fixing something that have no idea where the problem comes from.
I am checking this link here - the taillight is same as my car. and I am confused now in the link they claim that the taillight replacement is just a little light bulb - but I thought the red circle part is an LED light that cannot be replaced, the only way to replace a faulty part of that is to replace the entire module?