Problems with iPhone 3G and iPod Touch with Car Stereos

There are three cars in our family, and every car as a stereo with built-in iPod interfaces. Car 1 has a Pioneer system, Car 2 has a Kenwood, and my car has an Alpine system. I cant hear any audio when I connect an iPod touch or iPhone 3G to the Kenwood and Pioneer systems. They can perform functions on the iPhone/iPod touch, such as change songs, randomize songs, repeat songs, etc, however, there is no audio. 80% of the time the Alpine does the same thing; 10% of the time an error is displayed on the head unit; and 10% the iPod Touch/iPhone works perfectly.
All three systems can operate my iPod Nano correctly without any problems at all. Because the same problem occurs to all the stereos and because the iPod Touch and iPhone are basically the same minus phone capabilities, I am beginning to believe that the problem is with them and not the stereo systems. Has anyone else had this problem? If so, how can I fix it if it is fixable?
Mac OS X (10.5.4)