The amplifier is underneath the carpet in the passenger foot area. No need to remove the seat to access it, removing the cover on the console and the door jam will allow access to it.

Did you find out what the scoop is on the duplicate pins?

BTW, AUDIO SYSTEM CIRCUIT DIAGRAM goofed up in another way. The fuse (23) in the underhood accessory fuse block is HOT AT ALL TIMES. I found this out the hard way when I dug into the fuse block (I like my stereo to work without the key in, so I needed to keep the power amp HOT AT ALL TIMES). I am assuming it powers down when the antenna signal is disabled.

I replaced my head unit because I want to use a remote CD changer that will play MP3s (the stock head unit apparently will not).