If it is an earth problem, I would be concentrating on the rear light cluster earth, and the earth for the driving lights, as these are the two problem areas that you have mentioned.
I would specifically be looking at the Reverse Light problem, as these are connected into the main harness and thus into the gearbox harness through the reverse switching, or the pickup feed for the driving lights, as picking up the power feed for these items from the wrong point can cause signal change at the ECU, causing the ECU to do unexpected things
