Ray Transp Map?

I am trying to make a texture for a car’s wind shield and I’ve run into a problem. The material I am using uses ray mir and ray transp. In the past with many things I just used Z-Transp which is affected by the alpha of the material, so I could just add an alpha map and it would work fine to have certain pieces more transparent than others.

However alpha maps do not affect Ray Transp. Is there a different setting I shoud map my texture to? I don’t want it to affect the ray mir value at all.