I see you got your answer. I just got a pixhawk and want to use a dronecell (sim900) gprs shield for telemetry. Any chance you have the pinout for the pixhawk's 6pin? I believe I only need the ground, RX, and TX pins, which I will need to cross with the SW serial rx/tx pins on the dronecell. Thanks anyone who can help.
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Took some digging but I found my answer.
Red (furthest toward the "inside" of the APM) ---> 5V (red)
Black, 2nd from inside --> RX
Black, middle --> TX
Black, 2nd from outside --> CTS
Black, "outside" -- GND