It is common on alot of cars, that when the reverse switch is old, it will stick. It is a small plunger type switch, wwhich when you select reverse, pushes the plunger in and make a contact. When the switch gets all the old gammy oil round it it will stick on, giving you your Always On Prob. Change the switch (Approx £10) Had the same prob on my vectra, or take the switch out and clean it up. It might work. :D