Is it a big magnet to hold your wheel on because youve not got any wheel bolts in??
Seriously though, not a clue, id be tempted to say something to do with an alarm thats been fitted, what is the little groove in the end for, is it a button?
Well its a decent magnet if it is cos the wheels holding fine
its just a slot, got it recently.. the owner did say her boss had the car before her, and it has a mobile phone speaker and aerial thing installed in it, so maybe it has some kind of thing to do with the business use.
Either that or i thought about this, I have heard about them insurance trial schemes, where they have a box mounted and you pay as per how much and where you drive, maybe the slot has something to do with that.
i reckon it is an old style car fone! the slot looks like it would take an old credit card shape sim then there would ha been the phone up front in the cabin. do you have any extra aerials or holes in ur dash look under the back seats and see if there are any cut off wires.
or my other thought is that the government thinks they need to be trackng you beacuse ur a terrorist lol