First , INFANTRY must be involved (Armor vs armor never uses close defence regardless of terrain or if ambushing another armored unit).
Second, an ambush will force a unit to use its close defence value IF THE AMBUSHING UNIT IS AN INFANTRY TYPE (i.e. Blundering into an armored unit WILL trigger an ambush, but will not force the ambushed unit to use its close defence value.)
Now, if I'm correct in my assumptions (I know what assuming can do


