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PM me if you would like a scanned manual. You can run the GPS without the reverse wire. You do need to tap into the speedo signal though. Buy the ipod/iphone cable here: http://www.beckerautosound.com/parts/index_porsche.html
Does it take the place of the AUX cable? What kind of control do you have of the iphone from the radio, or is just the audio hook in?
IIRC besides the antenna there are two inputs, one from the Tach (speed indicator obviously) and one from the Climate Control (the reverse input - it's there as part of the feed that closes the rear HVAC intake when you are in reverse). I'm pretty sure both inputs can be bypassed and not used - the reverse certainly can as I don't have mine connected. If you want to install the antenna I found with a little trimming of the antenna base you can get it to fit under the front dash. It works just fine there and is completely hidden. There's a thread here somewhere that details that install.
Does it take the place of the AUX cable?
Yes. It's the same input used for the remote CD changer so it's an either or proposition - iPod or changer, not both. Not sure about what it controls but I believe it's just an audio feed so you need to use the controls on the iPod.