The UC5 is what handles that, which is why it is necessary for trailer surround. When it sees that the BCM is configured for VehConfig 8 aux trailer camera AND regular surround view, it will populate the Trailer Surround Camera and Aux 2 soft keys in the More Cams menu.
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If the UC5 sees the BCM as configured for VehConfig 8 aux trailer camera, but NOT regular surround view, then it will populate the Aux 1 and Aux 2 camera soft keys instead. The manual explains that regular surround view is necessary for trailer surround view, and if you have regular surround view, you can’t have both aux trailer cameras.