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Here's a pix of the rubber seal on the pass through on my 2015 wagon. Obviously I was trying to get the connector but you can see the seal behind it. It's just a thin push in grommet that just pops out if needed. Easy enough to remove and plenty of room to secure some screening over the hole to keep out bugs. Not sure how much road splash you might get from the rear wheel.

I don't like it at all!

There's a small hole on right rear area, aft of the wheel well, that I know of in the cargo version at least which you could possibly enlarge if you wanted to create an inlet underneath the vehicle. I use that hole for the sink drain though. Besides adding a secondary vent on the roof or adding a window or porthole along the doors or body of the vehicle, you could just use the driver and/or passenger windows with rain guards to mask the opening and keep out rain with the window(s) cracked. This is what I do, and I eventually added a mesh screen that I've taped magnets to along the perimeter which keeps it in place on the passenger side 24/7 to keep out the bugs whenever I crack this window for ventilation.