Volvo V70 auto estate 1999. Reversing light stays on while ingnition is on!

Most cars have a feature called a neutral safety switch. This device generally provides two functions: first it prevents the starter from being engaged unless the transmission is in neutral or park gear, secondly it turns the back-up lights on and off as appropriate. With age and use this switch can and does fail and more often it needs adjustment. Being a volvo part I do not imagine it is carried in stock at most parts stores, however a dealer's service/parts guy could show you the part, and its location on an illustrated parts breakdown. He also could give you some idea of how often this part creates problems such as yours. You could then make a decision as to whether or not you want to replace yours and whether or not you want to do it yourself.