I guess it all depends on what Display you have.  Old GS* or the new Style Display?  We have put Ag Leader/Trimble GPS on old GS* displays.  We have to use a cable from outback (OBK-2-GS2) that you will plug the red wire into pin F on the bottom plug of the GS* harness.  The other end of the cable is a 9 pin serial, thus allowing you to plug your 3rd party GPS into. Can't remember what the settings are supposed to be.......maybe 4800 baud,1hz?

Hope this helps,