Question Update camera parameter and post process volume

Hi everybody,
I'm quite new to Unity and therefore not deeply into the basics.
I was able to create a post process volume with a pixel shader, that undistores my projection screen depending on my tracked head position. Works well. Used to work fine in old style with native plugin.
My problem is, setting the camera position and frustum in post process step works, but as soon as I start the game, the player input overwrites this and I end up with the standard view.
Attaching a script to the camera and overwrite onPreRender / onPreCull isn't working anymore. Adding a onPreRender callback to the Camera class neither.
I need some way to adjust camera objects and do post processing per frame in a multi display project.
At the moment, my distortion meshes and eye-points live in my CustomPostProcessVolumeComponent derived class and is attached to CustomPostProcessInjectionPoint.AfterPostProcess.
Thank you