Hi Bernard,
Following your pictures you connected GPS TX line to pin 4 of socket P5. The schematic shows pin 4 routed to emitter of Q29. With a working transistor the signal has no chance to pass the transistor. Per mistake the Due receiver pin is connected to the base of this transistor...only a shortage between emitter and base lets pass your GPS signal line to the receiver pin of DUE.
That's my electronic understanding...
Michael