It's outlined in the thread you referenced a number of times. On page 15, you can clearly see in the DIY photos that the member installed a 3.5mm jack in his console that connected to the unit.

I was riding in a friend's new Hyundai last night and noticed her iPod cord was strange--it had a dock connector on one end and a 3.5mm jack and USB on the other (I assume using the USB to power and integrate, and the 3.5 to run the audio). Got me to thinking if something similar could be used on the unit to achieve what I'm looking for.

ref: Hyundai iPod Cable | eBay