Problems playing iPhone through AUX port in Saab

I cannot play my iPhone through from the headphone port through the AUX port on my 2007 Saab 9-3. I have successfully played my old iPod through the AUX port so I know it has worked. I have also successfully played the iPhone through the AUX port in another car (Honda Element). I have used three different cables, all of which are designed to work with the recessed port on the iPhone. The iPhone will play through two of my home stereos (out of the headphone port into the stereo AUX port).

I have discovered that payback is possible through the charging stand: by placing the iPhone into the charging stand and connecting the cable between it and the AUX port, I can successfully play music from the iPhone through the car speakers. So all is not lost. Still it seems odd that it does not play through the headphone port in the Saab, but will play through that port on two of my home stereos.