Right now Steering Wheel controls are non-functional for the G4 for all US cars, you can get around this right now by buying an SWI-X.
We are still working with Infill to get the integrated steering wheel controller
solution working for the US and support for the cars sold here. The hardware is there and hopefully will work for some people some day after just a software update. It will only work with cars that use resistive steering wheel controls. This will exclude all cars that use the CAN bus for streering wheel controls. The older your car is, the more likely it is to work through resistive controls. Even if your car has resistive steering wheel controls it may never work with the G4 as each car manufacturer implements thier resistive controls in a different way sometimes even varying between model and year.
The best way to tell if your car is capable of being supported by the G4 is seeing if it supported by PAC SWI-X. If it is, it is probably likely to work with the G4, however, we haven't been able to fully test any cars yet and I am really just making an educated guess that this is true. In any case, don't bother asking if your car will be compatible yet cause we just don't know, nor are we at the stage yet where we can begin compatibility testing and there is still a good chance that we may never be able to get the Streering Wheel Controller working for any US cars.
If things go as planned, there will be a patch for StreetDeck that will allow you to hook up your steering wheel controls to the G4. Most likely we will not be able to tell you how to hook up your steering wheel controls for your specific car and it will not be a trivial task for most people to figure out how to do it. The best we will be able to do until we build a steering wheel controller install database is tell you generally how they work. For example, my car requires adding an extra resistor in the right place to get all the controls to read anything.
If you buy a G4, the only advice I can give you until we can confirm there will be a patch for it is to purchase it assuming the integrated steering wheel controls will never work for your car and the only way to get them to work is to purchase a SWI-X