We don't have as much info for Magna's here. But on my 1983 750 there is a control unit called the brake and tail light sensor. It turns on a red light when any of the tail or brake bulbs go out for a safety warning. The manual shows that unit all the way up to 1986 700's also.
So maybe that is the same on the Magna and the red light means a bulb is out in the rear. Check your manual for that unit and check the bulbs.