relocated faceplate aux in jack gm headu

I have an aftermarket plug and play Sirius receiver in my recently-acquired 2007 Saab 9-3. The car has a generic GM head unit as stock. I really dislike having an audio cable entering the front of the head unit at all times. So, while I had my dash apart to install a bluetooth kit, I had the idea of adding a 1/8" audio cable to the circuit board where the aux jack connects. After removing the faceplate of the stock head unit, it was easy to see the the terminals for the jack. I used a multimeter to figure out which terminal was tip, sleeve, and ground (although there was a fourth terminal...?). I ran the audio cable behind the "ashtray"/storage compartment and into the back of my Sirius unit.

Something sounded not-quite-right to me when I play the Sirius through the head unit. First I wasn't sure if it was coming through in stereo, but I concluded that it was, through comparing left and right channels. However, the right channel is noticeably louder than the left. I am wondering if perhaps the left channel is somehow out of phase?

Does anyone have any idea what the problem might be? I believe the cable to be in proper condition.Thanks