Reactivating VW Sharan Rear Parking Sensor?

Recently bought a sharan which had previously had a towbar fitted to it (had it removed now) but the rear parking sensors do not work. From speaking to a friend, he says they have been deactivated because of the towbar.

Though I am curious as to how they would be reactivated. Is it most likely the dealers would have unplugged them or perhaps deactivated some other way?

Thank in advance.