after following some traces on the PCBs i found what seems to be the perfect spot!
this looks like the input to the pre-amp, if you look carefully you can see the single trace with the mono signal from the TEL input is split in two the TEL-L and TEL-R.
BTW: this PCB area is located in the main board in the bottom part. To reach it you just need to remove the bottom cover of the radio
as an initial test i used an x-acto knife to cut the PCB trace joining L & R and tested the radio. As expected, the on-star system, played only on the L side :-)
it looks like an interface to the iPod can be as simple as a cable an a switch!