The new amp is what powers the four full range speakers not the head unit. The only other outboard amp is the one to the front door subs and this is not regulated by the PAC Harness. The subs are powered by a separate amp which also receives a signal from the head unit through different connector. The head unit at this point is simply providing the output signal from its source (radio, CD, Aux output, etc.) to the OEM-1 which in turn sends it to the new amp. The head unitís signal is converted by the OEM-1 Unit to a low level signal (also known as a LINE LEVEL signal). You regulate the strength (amplitude) of this signal to the amp with the head unitís volume control (set initially at a certain level which I canít recall) and then ultimately by the output controls on the OEM-1. Too much signal from the OEM-1 to the new amp and you will cause distortion. How to regulate the level is another discussion, but it will just require some patience. Remember, at this point, the new amp is providing the power to the four full range speakers. The head unit is simply providing the audio signal to the sound system that the new amp will boost and send to your front and rear full range speakers. As I previously noted, all of the SYNC functions that I have tried continue to work as before. The PAC Harness provides all of the connections that the carís own harness contains. Therefore, it seems reasonable to expect that all features will still work. Every feature I used in the past (commands involving phone calls, music selections, climate control, etc.) has continued to work for me. Remember, this is my own experience with my vehicle that contains the Shaker 500 and the NAV system which includes Fordís SYNC system. I must admit I am not familiar with the "Door-Open Shut Off" feature. Hope this helps some.
Gramp you've taken a lot of time from what is probably a very interesting trip. Much appreciated.
One question I am still trying to figure though... Are the full range speakers powered by an amp integrated in the HU or is there an outboard amp? My concern is when the front speaker wires are returned from the aftermarket amp and spliced into the PAC. I am hoping the factory full range amp is in the HU and not "downstream" from the PAC. I could see going from the aftermarket amp into the original turning into a distorted mess.
Well 2 questions... Are all other functions preserved by PAC? Sync, door-open shutoff,etc.
Thanks again and enjoy your adventure.