Does anybody know what this is and where it goes?

looks like some type of under the front plastic, maybe under the rear end, in wheel wells?

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Where did you find it?

I bought two xk8s, one was wrecked in the front and the other was a flood car for parts.
When I finished having the wrecked rebuilt I sold it. I am now parting out the flood car and these were just laying on the back floor.

I have been searching for the plastic hinge covers on the outside of the seats. PM me if yours aren't broke.

Unfortunately, the seat hinge covers are gone, thanks anyway.

For what it's worth here is the JEPC description.

It's a trim that fixes behind the headlight, under the hood on the RHS of the car.