Without delving into the not very clear circuit diagram the only advice I can give, based on my own experience, is to check the wire loom at the rear boot lid hinge point, under the leather strap. I know it's the other end of the car bit bear with me. The loom gets crimped here every time the boot lid opens and closes, the wires fatigue and short out causing all sorts of strange issues. Really not sure if the headlight circuits link with the tail lights from memory but it may be worth checking. I remember the two tail lights are independently fused as it was always one side that blew the fuse. Your car is a similar age to mine so it could have the same degree of wire fatigue?
