the chassis ground is carried on the metal casing part of the DIN plug/socket - as opposed to the signal grounds which are carried on pin 2 (left and right together). The DIN cable will have, signal and signal ground wires, wrapped inside a braided shielding, it's this shielding that is connected to the 'metal casing' and carries the ground connection. This is why an amp fitted with 'din' connectors, may not have a ground terminal fitted.
The metal shell protrudes slightly proud of the pins, one benefit of this, over RCA connectors, is if you should connect/disconnect equipment with the amp switched on the ground connects first, avoiding the nasty buzz. Still not 'best practice' though.
You could connect your decks earth lead to the 'metal casing (shell)' of an adaptor, but as stated/shown in previous replies, if you can get one with the 'ground wire' fitted then it's a much simpler answer - din plugs are fiddly little things and soldering them is, for me at least, a last resort these days.