Mentall Ray And Maps

I'm very new to Maya. I'm currently working on a car texture.
The look i'm after is that of a muddy car. I have used Blinn and then added the dirt maps using layered shader, and this seems to work fine. Instead of the Blinn i want to use a Mental ray shader (car paint phenomenon) but i don't know how to use the dirt maps on a Mental ray shader. Can someone please give some advice!

Try here:

http://www.lamrug.org/resources/doc/paint.html