Our T5.1 had a button next to the aircon controls marked "rest". When you stopped and turned the engine off, you could press "rest" and use the residual heat from the engine to warm the van up. It was obviously instant and lasted about 20 minutes. I loved it, but the T6 we've just bought doesn't seem to have it. Is there a secret key combination I've missed?! Google suggests that other VW models still have the button.