[Elantra GT] LED Bulbs for Elantras - What Works, What Doesn't and Why

[Mark S. Hankins]

In short, you need either electronic rather than thermal flashers, and/or the addition of 
load resistors wired in parallel with the bulbs.

http://www.superbrightleds.com/carbulb-notes.htm

Failure to use the right parts will result in cruise control not working, etc.