The car used for Reed and Malloy's close-ups was towed by camera car. The car's windshield showed reflections of the camera and crew, so the car's windshield was removed. However, this led to the wind blowing Reed and Malloy's hair, so a shield was created to enclose the camera and front of the car.
The missing-windshield thing is
really noticeable on some shows. The Beverly Hillbillies did it, too, with every car in every scene shot on a soundstage. Windshield out, windows down.
Adam-12 reused a lot of B-roll, and I think that may have been a factor in their continuing to use the '69 Belvedere for an extra year instead of a '70--they had a ton of footage with the '68, shot during season 1, and they reused that with the '69. You can see the '68 marker lights in a lot of the drive-by footage.
