I have used the Apple iPod cable with no problem in the Elantra USB port with an iPod Nano & iPod Classic. You have to change the music settings, skip songs, etc. from the iPod which is distracting while driving.
I purchased the cord made by Hyundai at the dealership, that plugs into both USB and aux port. The music titles, album info etc show up on the radio screen, and you can use the buttons on the radio to change the tunes, view artists, albums... (safer while driving!). I've seen sellers on eBay that sells the Hyundai made one (cheapest starts @ $24+ w/ s/h - link here) http://cgi.ebay.com/ebaymotors/NEW-HYUNDAI-2009-09-iPOD-INPUT-USB-AUX-AUDIO-CABLE_W0QQitemZ230379648893QQcmdZViewItemQQptZCar_Audio_Video?hash=item35a3b2777d&_trksid=p4506.m20.l1116
, as well as copycat ones - I wouldn't buy a copycat one (my opinion).
Do you have a case on your iPods? I have experienced the "reading error" with the Hyundai cord on my son's iPod Classic (he has a leather case on it). It seems to do that because the plug going in to the iPod is not making full contact with the iPod because of the case. When he takes the case off and reconnects, there are no further problems.
Hope this helps, Good luck!