I have had my SLK350 on order(Â£1000 fee) for a few months now and I was wondering if someone could please advise how long before getting the car do you have to pay the full deposit(Â£4000 ish)to Mercedes.
I think yippym is right on this one, in that the money doesn't go to Mercedes itself but stays with the dealer. It's their way or raking in a little more cash to stash in their highest interest earning account while you wait for your car.
If you get a 'good' dealer then chances are that they won't ask you for more money, but how many of us know dealers like that? 
The dealers are being unscrupulous ( is that spelt right ?? ) and the deposit is nothing to do with MB. If the MB dealer is using his direct and own production allocation then this should be cost free, if on the other hand he is " buying" another dealers allocation, this may be the cause of the varying deposits changing hands.
I personally paid Â£1500 deposit with nothing else until the car was in the showroom and checked over.
The danger is that, if the dealer for some reason is late with the car, they really have got you where it hurts.
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