Maybe just wire tap into the wire that feeds the brake lights and use it to trigger a 5 pin relay.  Not sure if the trigger wire would see voltage if you had a power source hooked up to a switch also feeding the strobes at the same connection as the power coming from the relay, so you could just use a diode on the wire from the relay to keep it from back feeding when you use the switch. 

That made sense to me, lol, I could be way off though?