East detecting Resistance/West

[roninmd]

Hi, I need to be able to play music when East units detect a Resistance or West unit. At the moment I use  a giant trigger, but this only plays once as
the triggering unit needs to leave the area of the trigger in order for it to start again at the next detection point.

Is there a way to do this so that each time an unit detects an enemy unit this music kicks in?