Well, I finally found it while working on wiring up the video system (Arbitrator) in the console. The SYNC box is bolted to the bottom of the console, with the plug facing toward the passenger side. To get to it, pop off the trim around the front cup holders and around the shifter. Then removed the top paneling there that runs from the dash to the big center arm rest. There are then two screws holding the front of the console down. Remove those, along with the 4 screws on the sides. The front two are behind those little plastic covers and the rear two are exposed (just slide the seat forward and access from the rear) Then just lift up the back corner of the console and plug in your USB! I accessed it from the rear passenger door with the front passenger seat all the way forward.
Its nice having my iPhone wired up all the time and the primary USB still available for hard drive, thumb drive, or flash updates.
I really like it. I don't use any of the voice stuff, just the iphone/ipod interface and the bluetooth phone. I've heard getting the "add on" kit is not as good as the original. I would suggest spending the $550 on a good bluetooth phone kit and an ipod adapter.
I wish (yes this is a side note) that there was more MP3 accessories than for IPOD. Most all quality MP3 players use the open access communication protocol that windows media player stores music by. Even if you don't sync your music with WMP, an MP3 players OS most likely syncs by the same protocol when you turn it on, just like almost every digital camera stores images in a folder named DCIM. It should be easy for a carputer like the SYNC system to have just as much control over a normal MP3 player as a IPOD/IPHONE/ISUCK :P