I would do it like this:
Create a switch trigger that fires when the bolean condition radioTransmission (or whatever) is set to true. Use the trigger to start the Medvac chopper in the usual way (synchronize). Now you want to create a way for any player who gets close to the car to use the radio.
In the init.sqs of your mission you add the line:
publicVariable "radioTransmission" ; radioTransmission=false
This ensures that every machine, not just the server, in the game will be told whenever the variable radioTransmission toggles between true or false. Then, in the init line of the patrol car your write:
this addAction ["Use Radio", "radio.sqs"]
This gives any unit who gets within 3-5 meters of the car an action named "Use Radio". Then make a small script called radio.sqs that looks like this:
radioTransmission=true
exit
Now you have set the variable radioTransmission to true on all computers in the game, enabling anybody in the group to use the "radio" by aproaching the car.
Off course this will be a radio transmission without sound or texts. But they can be added without to much effort if you have sound files to go with your mission.