That's a Deutsch DT-series connector. Commonly used in lots of heavy equipment and truck applications. Your local Caterpillar dealer parts dept will have any parts you need. A complete kit with crimp tool is available from Amazon:
My personal opinion is that the Deutsch DT connectors are a bit heavier duty and easier to take apart than Weatherpack, but it's also what I'm used to dealing with regularly. Weatherpack is good quality stuff too. Either is a professional solution in comparison to the worthless butt splices and scotch-locks included in light kits which are guaranteed to fail in the middle of nowhere.