Hi Matthew,
I think I see your confusion here.
If you are using a parallel encoding altimeter Plug 9 pin 2 isn’t available for “GPS Enable” function as it’s used as a data line. So you must ground Jack 8 pin 2 instead.
In Section 2.6 “Settings after Installation” of the Becker Manual, you must select, through the menu function, the type of Encoding altimeter being used. If you select “Parallel”, which is the default, the data line P9 pin2 is presumably reconfigured away from its use as GPS enable to altimeter data functionality. It would probably be a good idea to confirm this with Becker Support, first!
NB. I used Issue 4 of the manual dated June 2016, I see you have issue 5.
Thx,
Chris