The wiring diagram is a start but it will not be that easy. I installed steering wheel controls to my Axxera AVM70LH radio but added a Chinese made steering wheel controls to the Ford Steering wheel which did not have any, other than the cruise. I documented it here in this thread.
In that process I contacted Dual who directed me to Mito who support that model of radio. After asking some questions, I determined that the Tech support person was not going to be of any assistance. I wasn't going to give up so I sent Dual an email specifically asking what interface to use with the radio, and as of this date have not heard back from them. After all they are glad to hear from me and will get back as soon as possible. What is "possible"? Anyways in the mean time it is working but I am not going to let them know. One of the questions I asked the tech was would I receive a message on the SWC screen of the radio. He avoided answering. Basically you need to completely remove power from the head unit, restore power, go to the SWC screen, match all your controls, then press enter.
What you are facing is they need to hook up to the existing steering wheel controls of your MB sprinter and interface to the head unit. In order they need to connect to the wiring harness capture the signals then transpose those signals into what the head unit will understand. From my research there are basically two companies that build interfaces for this. They are PAC-Audio and Axxess. These companies are in this business. What I found at both of their websites, is they have interfaces to the harness, but they did not list any Axxera radio. In my case I did not need a connection to the harness as the signals were coming from my add on buttons.
Hope this helps you understand and good luck to you.
Just curious, did all the sprinter based units come with MB steering wheel controls and used the Axxera radio? Or did they use a MB factory radio or did they not have steering wheel controls? Maybe Thor installed and interface that is not working?