Installing a 3.5mm jacking into Pro2

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I have a very small 3.5 jack. I think I can fine space to install it in my phone and then thinking of sending the wire into the usb port at the bottom.

I have this info:
http://pinouts.ru/Home/Tele35s_pinout.shtml
http://www.tracyandmatt.co.uk/blogs/...tor_pin_config

My question is, if I just connect the wire up. How does the software know there is a headset attached to the phone? or will it always think there is?

it measure the resistance between ground and left and right
a normal headset connected would maybe give 8ohm where no headset connected would be infinity ohms

thanks.

does it matter which gnd wire i use? I notice there are 2 different ones