If you are pairing it to a specific trailer, make sure you change the bulbs in it to multi-voltage LEDs so they will run of 12 or 24V the same, otherwise you'll either blow the 12V bulbs when hooked to the HMMWV, or you'll have extremely dim bulbs when hooked to 12V and using 24V bulbs.
If all you want is a straight connection, then there are a couple choices, but they're all in the $130+ range from what I see. If you can locate a spare 12 pin plug somehow you can probably wire one up a lot cheaper.
If you want to be able to tow any civi trailer without modifying them, you want one of these:
Yes, you can build one for cheaper, but the frame mount version is what the military uses for a lot of their 5 ton semi tractors and the like, so it's good quality.