How to control Ambient Lighting and Spherical Harmonics

I am trying to control the Spherical Harmonics and the Ambient lighting in my surface shader but I don't find a way to do that. I've discovered that they can be disabled by using

• noambient - Do not apply any ambient lighting or spherical harmonics lights.

• novertexlights - Do not apply any spherical harmonics or per-vertex
  lights in Forward rendering.

And you can directly access the current ambient color with this : UNITY_LIGHTMODEL_AMBIENT

But there is no whay to control it. If I use noambient or novertexlights and try to apply UNITY_LIGHTMODEL_AMBIENT manually in the lighting function, the value is not filled and I get a simple grey color. And if I don't use noambient and novertexlights I can't control the ambient lighting, it is applied automatically.

Is there a way to control it ?