FPV camera Switch fail to operates with Pixhawk clon (HK32)

Dear folks: I have the HK32 a Pixhawk clone, the autopilot operates perfect. Now I want to include 2 cameras (Run Cam and a mini FPV camera pointing backwards, all in order to have a 360° view of rje aircraft. The problem is, that I can not achieve the switching operation of the switch. I connect the camera switch to the chanel 12 on my 9XR transmiter, the receiver is the UHF mini from Immersion, transmiting in PPM to Pixhawk Clone (one single cable), and the receiver is 12 channel PPM capable. But the thing is, the switch does not asnwer to my knob (ch12 operator) and does not change between front and back cameras. The only way to obtain some results is to change the Pixhawk 12 RC Ch to 21, lets say to a Rudder imput, so when you move the rudder stick to the right, the camera changes between the fron and rer cameras. But I do not want to link my switch to the rudder stick, I want to link with rhe P3 Knob, how to do it???

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What happened inside Pixhawk when you asign a RC function like 21 (rudder) using MP?

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