Car radio ad. That is, it's a radio ad for a car, not an ad for a car radio. . .

Confusingly, this ad would have been heard on a car radio, which would presumably make it a radio ad in a car for a car on a car radio. Now due to the fact that the radio would break under the weight of a car this obviously doesn't mean that the car was on the car radio at the time (although the ad for the car was) but instead the radio with the ad for the car on was playing in the car on the radio.

Probably.