No reverse light when the front lights are off.
Reverse light when the front lights are on.
The reverse light should function anytime reverse is engaged irrespective of the lights being on or off.
On the reverse lights situation, there's a 2 pin plug just under the ignition barrel if that's fallen out it will cause the symptoms you're seeing. Have you had the dash pod off recently, maybe it got knocked?
see also http://jenniskens.livedsl.nl/Technical/Tips/9/MyTip943.htm