Also have a GS2 system. Have been running accutrak autosteer with a RS232 signal. The newer harnesses have a 9 pin connector half way up the harness.

Don't know about getting the signal from the back of the display.

I currently get the signal right from the receiver

 On the 12 pin connector: (for laptop connection)

       pin 10--- is RS232 RXD connect to pin 3 on a DB9 connector(for laptop)

       pin 3---- is RS232 TXD connect to pin 2 on DB9 connector

       ground is connected to pin 5 on DB9 connector

Set NEMA string output of receiver ,and your set.