Chris,
The "closed ones" are of two styles......built that way out of the sheet metal so that there is no open option without getting violent with a cutting wheel! ....and made with a separate cover piece (as the guys mentioned above) pop-riveted in place, with a drain at the bottom that routed water over the wheel well and out....
You may find the hose tucked into the cover recess around the wheel opening! That's where I found mine......no covers.....just hoses! Not efficient......
If you check out your frame work around these vents, look for tiny rivet holes in the ribs of the under works. It is likely that yours may have been removed. I hadn't heard of any cars not getting something to cover the vents........doesn't mean that a few didn't slip thru....missing them.
They don't really vent much in my opinion......just look good!!!
Cheers!
Steve