you need to have the steering wheel as a separate object, then you have to set the pivot in the middle of the steering wheel at the angle at which it is supposed to rotate.
i'm not entirely sure if it needs its own cdt (haven't figured that out yet) but i added one just in case, just a basic lowpoly cylinder as subobject with the same rotated pivot.
then you need the following line to the classes XML inside <TruckData>
<SteeringWheel Frame="xxx" />
while xxx is the name of your steering wheel object. at least that's how it worked for me