Also 90% of modern merc's need a part called a CAN-BUS interface in order for navi's to function properly.
Basically the speedo signal in most merc's is a can-bus type signal via 2 twisted cables (can high & can low), and unfortunately navigation systems do not yet speak this lauguage. The speedo signal input into the navi needs to be a square wave. The can-bus interface is basically a translator, that effectively conerts the can high & low into a conventional square wave output. This means your navi will work quite happily, as its seeing the type of signal it needs.
Now are you sitting down????
CAN-BUS interfaces are £149-00 mate.
There is a slim chance you won't require one, but i wouldn't know until i've had a look at the wiring harness in your car.
I've been off work for the past few days so i'm not quite sure whats booked in this week until i get to work in the morning.
If you want, then gimme a bell tomorrow at some point, or call round if your passing by
Cheers, (and i hope the above makes sense lol!!!!)
Brian.