pac ms frd1 and 4 channel amp

https://www.the12volt.com/relays/page5.asp

On the above page there are several diagrams with a quenching diode paralleled across the coil of a relay.  A diode is a polarity sensitive device.  You need to put it across only one of the relays.

Use the last diagram I posted for the relays.  The other guy pointed out that I had the 2 inputs reversed in the first diagram. 

I will read your long post later today and then answer any questions you have.

The relays are the only way to do what you need done.  A diode will not isolate the amp signal from the sync module.  A diode in line with an audio signal will not sound good at all. 

I too was shocked when PAC tech support told me that I could not run an amp through their module.  The very first piece of PAC equipment I ever saw was an SNi-8.  A line out converter for an 8 watt radio.  They got their start interfacing OEM radios to aftermarket amplifiers.  Why would they make a piece as this that would not allow you to use an amplifier?