You might have the wiring in already for it - easy to spot as it is a big connector that would not have plugged into anything else (as you have the clock).
I think you will need the indicator stalk with the button to cycle through the functions.
Can't remember as I ditched mine for the round clock instead a while ago - the OBC looks out of place by comparison I think - remember all good quality cars have an analogue clock (Rolls, Bentley, Maserati etc)!
I was going to retrofit mine into the space behind the ashtray, seen that done before ( a square on though not round).