Regarding the interface of preamp level audio sources to a 2003 GM Class II factory radio
without a slave unit: I just received a message from VP of sales, Robert Wilson, of PAC and he advised that they are "working on it right now actually". He further advised that we are "probably looking at between 60-90 to be realistic" and to check their website for updates.
PAC Website Link
Speaking for myself, it can't be finished too soon!
Ed