Can't help you with the "Lock" situation on the deck, however if you remove the radio and the cassette holders, you will have a double DIN space, but due to the shape of the opening, you'll have to get some sort of adapter piece.

As a truly interesting extreme, a euro 190 owner just recently had an MB Comand unit installed in the space, so now his diesel has GPS with voice instructions, CD and the inputs for video!