Chevrolet Lumina APV Radio Code

I recently replaced the radio/head unit that was installed by the previous owner. It wasn't the factory head unit, just a sony casette player. The aftermarket radio adapter harness was already installed so it was a seem less install. The radio unit powers on and is able to pick up reception, however it can get extremely distorted with bombarding static noise at times. When this occurs I'm not usually under power lines, trees, or any other cause of static interference. I believe that it is the antenna itself, I just need to know where the actual Antenna itself is located within the van. I have checked for an exterior antenna, but had no luck. So please answer my question of the Antenna's location.

Radio installation