Perhaps you need a different dealer? Sometimes, though, if a car is not doing what it was doing, it is impossible to find the problem. At a minimum, I'd check botht he switch and the blinker for the four way flashers. Perhpas once the temperature drops to a certain point the flasher kicks in?
Very doubtful they'd pay additional costs. You may, however, continue to have some coverage since the problem was reported, never diagnosed, and returns. You may want to go on record with both the dealer and Hyundai office that the problem is unresolved.