Mine's the Exec Tourer - pre facelift. If the front screen rain sensor is detecting moisture and you pop it into reverse, the rear wiper will go continuously, weather your rear wiper is switched on or not. Once in normal driving (forward facing gears ), even with the fronts on auto, the rear will need to be switched on to operate.
At least that's how my car works. I imagine it's the same across the pre-facelift models, maybe the same for the post facelift too.
It's a Sport tourer pre-facelift. I presume it doesn't have auto wipers as there's nothing on the stick that says "auto". Perhaps the rear one comes on when selecting reverse?