Had an idea about how to wire the two way indicator switch.
It will be connected via training wires to a DB9 connector plug. When making up the plug, each wire (3 of) will be connected to 3 pins (thus using all nine connectors)
The corresponding socket will be mounted on the dash assembly. 

The pin out will be as follows:
1. L ind out cable to lamps
2. L diode out for buzzer 
3. L diode in from Hazard
4. R ind out cable to lamps
5. R diode out for buzzer
6. R diode in from Hazard
7. Common 12V ind feed
8. 12V feed out to Hazard switch
9. Spare 12V feed

Thus they are grouped in threes (L, R and Common)

Although it makes the wiring run a little longer, this arrangement centralises the electronics and makes the switch a remote item. As the switch is in a harsh environment, the electronics it controls are far removed from the potential of damage.

That’s the theory anyway. I now have to show how it works in practice...