2008 avenger no radio signal

We had this car in because a guy wanted an aftermarket radio. We sold him a Kenwood unit (08 unit) and the radio will not pick up any stations with the factory antenna. We tried a diferent antenna adapter and nothing. I then went ahead and tried a different Kenwood unit (07 unit) and it was the same. I suspect it's the antenna but the facory radio picks all the stations in the area and even the hard to pick up. The funny thing is that when the problem ocurred he got mad and went to Best Buy and they told him that  he was crazy for changing the radio because you can't change the radio on this car.

The guy is comming back today to get the problem corrected but he doesn't want to change the antenna because it works with the factory radio. And I need a solution for this problem. So any ideas are welcome.