How do you have the GPS card connected? Is it between the ethernet shield and the Mega? The SPI data lines are connected to the shields by the ICSP pins, so the ethernet shield's ICSP pins are too short to put the GPS card on top of it.
How do you have the GPS card connected? Is it between the ethernet shield and the Mega? The SPI data lines are connected to the shields by the ICSP pins, so the ethernet shield's ICSP pins are too short to put the GPS card on top of it.