When unmounting one of the door panels of my xk8 1998 i found a loose fitting which apparently can be connected to a kind of speaker under the triangle cover above the door panel
Can anyone explain to me what kind of speaker can be mounted at that location?

 

That connector is for a tweeter unit mounted in the door 'sail', but if you don't have the premium audio system (amplifier in the trunk above the CD player, subwoofer in the parcel shelf if the car is a coupé), then the other end of the wiring doesn't connect to anything.