my 850 had a switch that had failed, the main switch had charred contacts, you could try spraying contact cleaner in there. If you have major paitence you can take the assembly apart but that leaves you reasembling the actual switch toggles. Im however not sure what you mean by rear lights, do you mean they just sit on all the time and the front ones function? Or all the lights stay on. If you clean the switch and that doesnt help you might try looking at all the door contacts. My guess would be the switch tho. Other than that those are the basic parts to the lights, if they go on, it has to be a switch somewhere that is mesing you up.