Hazard warning switch

As per the topic. Finally got round to doing the remote central locking extension to the front doors. Hatch works great ,so I'm re doing the power supply unit to make it ignition on only working, and adding an indication using the hazards to say that the car is locked. Simply the lock sets off the hazards ( which for anyone interested is a toggle arrangement-a short sets the hazards off ,and another short (on pressing switch ),stops them. Simplest solution is an opto coupler across the warning lamp circuit, taking approx 11mA  fro what Haynes calls the "electrical control system unit". The other option is to tap into one of the indicator circuit lines ,which is a bit more difficult. Anyone care to comment on the dangers of  connecting across the hazard lamp circuit.