Howdy!. I'm working on this project. It is a VW samba bus from the 50's. The drawing is seen in the attached picture.
The part which I'm struggling the most are the air ventilation grills seen in the backside of the bus.

I don't know how to create that shade or hollow effect as it is depicted in the sample. Because the original file depicts this part some blurry I cannot distinguish exactly where is the shadow or how the light enters in the object so it creates that hollow effect.

Can someone help me with how to create this or emulate this?.

In my research by comparing with pictures of samba VW buses around I found this samples of engine vent trims.
But from the different examples it seems that the vent trims protrude or stick out from the surface. Is it possible to use these ideas the replicate the part which I'm struggling?.

Can someone help me please?. I'm stuck with this problem.

Hi.

Funny some models have vents inside as dent and some as bump.

Maybe like something for the "dent"-version:

Here's my simple take.