The reason there are no other wires or connectors behind the stock radio in your base model is because Honda uses an entirely different wiring harness in each of the models. I'm sorry to say that the NAV unit you have obtained is useless except as a replacement for another NAV equipped model.
Besides the steering wheel controls (for both volume & station/track selection like the EX models) the NAV unit also has steering controls for voice activation of navigation and Bluetooth telephone calls. There is also a microphone up in the map light cluster. Also, like the EX models, there is a USB connector inside the center console.
My suggestion is that you eBay the NAV unit and buy a double din head unit made by Pioneer or Kenwood that will be much easier to adapt to your vehicle. Metro makes adapters for the stock connectors and they or others make the double din surround. Actually that part is also available from Honda parts. There are threads about that here. Believe me, this has been discussed, not necessarily here, but on other Honda forums. There is no way to make that unit work without tearing apart your entire car and replacing the wiring harness. If it were possible it would have been an available accessory and not a different model.
I can also tell you, having bought the NAV model new in 2010, it not worth it. We only did it to get Bluetooth. A NAV without real time traffic is useless. That and the constant need to buy new DVD disks to update the maps. I have everything on my phone. I just need to have it display on a big screen now.
The latest units now have Apple CarPlay and the Android equivalent. (That also means that the previous models are now dirt cheap)
Sorry, hope can get your money back.