I had a touchy backup light on my 240. The problem was the connection to the reverse switch on top of the transmission. (M47) There is a connector there which can pull loose. All I had to do was push the connector in tighter and the lights worked. The M46 is the same but the automatic back-up switch is up on the selector, I believe and this switch can get out of adjustment. Which tranny do you have? Of course the switch itself may be the problem or you could have bad wiring. Because the lights come on in reverse uphill, I'd guess that the problem is a loose connection somewhere.