adding aux to stock hu 2006 pontiac g6

Hey guys, Great forum here! First time posting. So instead of buying a $190 addon for this stereo unit, I decided to hack up an aux cable on my own. After hours of messing around with the XM addon port, I figured out that I can use the pins right below the XM ones for a second AUX. So I hacked up my aux cable, and on the bottom row, pins 2-4 I have black, red, and ground. Well this absolutely works the IPhone sounds GREAT coming through the newly added AUX. So here's my dilemma, where I was hoping for a little more expertise to help out. I am looking to retain the XM also. So in pins 2-4 on the top row, I have maintained the XM connections. If i connect either the top row XM or the bottom row AUX connectors, everything works great. When I connect all 6 wires, the sound goes all crazy. I believe it is because, even though I have them as separate pins on the back, in actuality the stereo is sharing their ground. If I remove either ground, the problem goes away, or if I leave both grounds in and remove one set of L/R the problem goes away. I was really hoping to avoid having to put a physical switch in. What I'd like to be able to do is set the XM to 000 (radio ID) and use the aux cable, and then just use the XM when nothing is going through aux. I'm so close to making this work if not for the shared ground. Right now I'm thinking I have to put a spst switch on the ground (or switch the L/R, one or the other) and hide it in the glove box or somewhere. Like I said I REALLY don't want to have a manual switch. Can anyone think of an alternative, or something I may be doing wrong?

What happens when you unplug your iPhone with all the wires connected? What happens if you mute the sound coming out of your iPhone with all wires connected?