rear wiper

hi i have a 2004 l322. when the ignition is on the rear wiper wipes about every twenty seconds (switched off) but if you start the car it only works when switched on any ideas?

It's not an automatic by any chance?
Just wondering if it's something to do with the wiper coming on when you select reverse but the switching being confused.
I know that this should only happen when you have the front wipers on but - stranger things have happened.
Try moving the gearshift with the ignition on.
If it's a manual ignore everything I've said.

yes it is auto and ill try that you never know