The RCD500 is OK for the age of the car - it was an option at the time. The CAN-BUS issue affected older cars that weren't designed to have the newer RNS stereos installed. That said, there were issues with RCD 300 / 500's causing battery drain but it wasn't the CAN-BUS issue it was becuase the stereos would register phantom CD's being installed in the middle of the night, turn themselves on and drain the battery. If it was bought of ebay or the like that could well be the reason it was put there.
As said though, disconnect the denison and take it from there.