I heard about movies are broadcast over radio, what frequency where to tune in at Delsea drive in. When I was growing up they use door mounted speakers.

Yes they tell you what station to turn on to listen to the movie

They do tell you when you get there. They also have radios for rent if you don't want to use your car.

They'll tell you when you pay for your tickets

When you pay they tell you what station to tune in to for that screen

They use two different GM frequencies, corresponding with the screen you're watching. When you purchase your ticket for the movie, they tell you which screen to drive to and what station number to tune your radio.

Not sure ask wen you go there they will tell you.