I connect my phone to my car audio cord and this comes up "This accessory may not be supported by this iPhone."

I had a iPod kit installed in my car in May, and it has worked perfectly till recently. **disclamier: I got my phone wet like 6 weeks ago, but it works fine** Over the past month, everytime I put the cord(looks like an iPhone 4 charger) into my phone it won't connent to my stereo and music would play from my phone instead. Then that little "this isn't supported" pop up comes up. So I'm not sure if my audio player is messed up or if its just my phone and I need to get a new one. Thanks!