1998 XK8 Convertible. Working on other things I ran across the plastic housing shown in the picture left of the spare tire and below the fuel door. The cover was not snapped completely in place. I don't see any wires, tubes, etc. attached to it. Trying to see what is inside with a mirror it looks like there is some thin rubber flapper material inside. What is it? No idea what to use for tags.

 

It's a vent for the trunk. There should be a matching one on the other side.

Not the best idea ever as they allow road cr*p to get lodged in there so the lower part of the rear wing rots out, unless you're fortunate and live in a salt-free zone...

 

Some kind of vent is all I could think of, but couldn't come up with a reason it would need one. This is a fair weather vehicle only and stays in the garage otherwise so maybe no harm done yet.
Thanks

 

There to relieve pressure.

 

John from To The Garage has a segment on this. Starts at 5:30 and looks at it from under the car and then from inside the trunk/boot.

 

Mine is always filled up with water, it's a crap design in a crap location. Like michaelh said it's for pressure difference when closing the boot or doors although I'm pretty sure Jaguar could have put it into a better location then behind the rear bloody wheels.