That's a NATO 12-pin recepticle. There are lots of pinout diagrams for these on net. Here's one that might mach up:
A simple check with a multimeter will tell you if that's correct.
What I would do is to figure out the pinout for your trailer's cable, then compare it to the NATO pinout. I would buy one of the male NATO cables and chop one end off. Then I would get waterproof heat shrink buttsplice connectors and splice it together with your trailer cable. As you know, military trucks have many lighting selections depending on the tactical situation. Pick the lines that match up with your normal nighttime lights setting. Again, the multimeter will help you here.
------ Follow up post added March 31st, 2013 05:30 PM ------
Also, check the voltage on the pins. They may be putting out 24V, which would need to be converted down to 12V.