Make the triggers activate repeatedly when the player is present. In the On Activation field, put:
AllowRadio = true;
On Deactivation:
AllowRadio = false;
Then, in the radio scripts, have this line before the rest:
?!AllowRadio: exit
I would also suggest you initialize AllowRadio by putting the following command in init.sqs:
AllowRadio=false
-Pilot