Dirt lens effect on a surface with transparency and blend mode.

Hi everyone,

I want to create a transparency windshield on a vehicle. The should look from inside to outside.
The glas should have 95% transpareny with cube reflection from inside.
Now I want to have a dirt glass effect, that came visible while facing the sun direction.
(As a dirt lens effect)
Unfortunately if the sun is in front the blend mode areas become dark should the ambience light.

See real example:
https://www.youtube.com/watch?v=Jz9yAPLnNTA
I don't want to use a post image effect because that completely unrealistic.

Does anyone know how to set up shader forge in case of the sun direction or normals?

Thank you and kind regards,
R.
(Uthe unity Shader forge forum is closed, where to ask then?)