As example this audi:

And this grill:

You select the mesh and move it to the picture of the audi, and make it smaller with edit->free transform:

Lower the opacity of the grill layer and select the part where the grill has got to be:

Select->inverse and edit->cut. Now go to layer->layerstyles and choose inner glow
and the following settings:

Lower the opacity back to 100% and ready: