Just replaced the door on my E420 - good advice above - likewise only necessary to ease the cover open a little to fit the new door - not a big job. General Disassembly instructions above state "Remove the three screws from the card compartment"; there's only two on the E420.
Just for anyone that hasn't realised, I had to change the door because I kept getting a warning that the card cover was open. This was caused by wear on the card door latch.