nowadays, duplicating car keys aint as easy as going to the Mr
Mint or something like that and voila. Modern keys have tiny
microchips embedded in the key itself... the sensors inside the
keyhole of yr car picks up the signal from the chip, varifies
whether it is the correct one then it will allow you to start yr
car or open the door. Usually require coding of such keys with the
manufacturer's diagnostic tester or system to match with the
car.
As for your Mercs infra-red key... if you can afford a Merc, you
should be aware of the cost of advanced technology found inside and
along with the car. 
