In this tutorial We'll teach you how to airbrush vents... it's very easy, you just need to practice!
Next, select the area you want to be a vent, and make it a new layer. Make sure that is in perspective with the car or it wont look good!
Now with the vent layer still selected take out the polygon lasso tool and hold down Alt so you can deselct some of the vent. We do this because we will paint here and give the vent a 3d look. leave just a little room to paint in, dont make it too thick or it wont look real. Also on the bottom, you will want to add to the selection a little (look at the pic and you will see what im talking about) now make sure you click on the backround layer and then make a layer. now drag that layer above the vent layer.
Now switch to the actual vent layer and get out a SOFT edged eraser, erase any parts that you feel u dont need.
Now merge those 3 layers into one. Make any nessecary adjustments with the size and shape by transforming..