I did what you say earlier, and here is the results.

I measured every cable, but they all seems fine, so that isn't the problem. When I measured the audio input on the radio module, it was around 700 ohms. I also tried to switch between ground and signal and noticed that the sound are exactly the same. I can hear the sound from the video, but no different from when it's connected as it should be. I found this a little odd. Could it be that the video module is defect or something? The nav sound that goes in to it are all ok.