OK I have an update on this. I have been doing a lot of searching and reading and thanks to dpnvektor who has a link to electrical diagrams in the automatic climate control thread I think I have an answer. To complete the steering wheel audio control mod you need to purchase the control assembly 36770-SNA-A11 (2006) or 36770-SNA-A12 (2007). I don't think there is a difference between the two parts but they have different numbers. Next you need to install the 2 wires missing from vehicles without steering wheel audio controls. Basically you need to run 2 wires from the connector at the steering wheel to the connector at the audio unit. Here are some pictures of the connectors in question.
Steering Wheel Connector
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Audio Unit Connector
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The view of these connectors is from the back. So following the DIY posted in the link above you would need to remove the upper portion of the steering column shroud and the portion of the dash surrounding the audio unit/climate controls. Instead of purchasing the EX wiring harness and cutting/splicing wires together you only need to run the 2 wires shown from one connector to the other. This is all in theory for now as I haven't had the time to attempt this yet but I see no reason why it won't work. If anyone attempts this before me let me know how you make out.