No diffuse channel used, no proper viewport display (so far).
We all would like to have this changed in the future but for now it is just the way it is.
There is nothing you can do about the issue at hand. Car paint is reflective and does not have a diffuse property. You can switch the diffuse on in a material to see it better in the viewport (and switch it off before rendering again), you can use the IR a lot, but in general, you have to wait for a fix from the coders.