Yup, I missed this thread until now, but that would have been my first guess. If you push hard enough you can plug it in backwards, which is why I added the following line on the instructions:
Be sure that the truck harness connector is oriented so that its latch engages the angled block on the Rail Pressure harness connector, not the non-angled block on the other side of the connector.
Regards,
Michael Pliska