[Artem F]

imho, it's the turn switch. The upper brake light is wired directly to the brake switch. The lower ones go through the turn switch because the same bulbs are used both for brakes and for turns and one has to cope with it somehow when the car brakes and turns simultaneously. Does it help if you move the turn switch rod up-down-left-right-forward-brackwards a little? If it does then the turn switch definitely needs replacement ($60 plus one has to borrow two pullers, one for the wheel, one for the wheel plate or how is it called)