The '07 S has one stock. I had been doing the exact same thing as you until I got my D+P. The problem is, you have to look down at the iPod to change anything, taking your eyes off the road. I had one close call, where I looked down a bit too long.
Now, where did you tap in to? On the stock head unit in mine, I had to fool my stereo into thinking there was something connected to it by using a resistor and tying a signal line to ground. The stock aux-in jack did this inside the jack, but since I didn't want cables running out of my dash and plugging into the jack, I had to reverse engineer how it worked.
If anyone cares, The plug on the back of the stereo has 5 wires.....grey, green, 2 red, and a white. The green and red closest to the grey are Right and Left +ve and the grey is ground. I'm not sure what the other white is, so I left it alone. But you need to connect the other red wire to the grey(ground) wire through a 3.3Kohm resistor. This red wire is the signal wire that tells the stock stereo that something is plugged into the aux-in jack. If this isnt done, then pushing the aux/cd button will do nothing and will not switch the stereo over to the aux-in.
This is the display mounted in easy viewing, ie. I dont kill myself.