Do you speak spanish?
Sorry for the late reply. We are very busy getting ready for Maker Faire in San Francisco, California.
To answer your question:
Pin 18 is disconnected. From looking at the schem for the GPS chip you could solder a resistor between Vcc_RF and V_ANT, but this would be under your own risk, and it is dangerous. You could permanently damage your GPS.
We will put in a small jumper to enable users to use an active antenna in the next revision of this shield