problems connecting MacBook to SmartNav for audio playback

My daughter is an over the road truck driver. she recently purchased a new Peterbilt with SmartNav. She attempted to cable her MacBook to the AUX port via a 3.5mm mini phone cable, but the audio only plays one channel. She had the shop investigate and they told her that she needed an adapter and pointed her to http://www.kvconnection.com/Default.asp which describes other 3.5mm cables, but doesn't specify which to get.

Are there other options available, such as a USB to USB audio out? Or a specific adapter cable that she should get to connect her MacBook to the SmartNav systerm?