Yep, that purple wire is on mine, too. If you have yours around, could you verify that the blue is 12V for me? I'd really appreciate it.

So, after following wires in my wiring harness, I have discovered that the Amp Remote line is wired into a dark green line that disappears down the other serial port. I have to imagine this is the power for the other serial port.

This is probably why the amp sense line never worked for me when I hooked it up to the amp in the car - it must be broken somewhere. I ended up just hooking that amp up to the 12V power supply to get it to turn on when it was still in the car.