My comments are based on the assumption that we are talking about the M211 made by GMC for the military. If not, please disregard all.
My experience is on the M135CDN, basically the same as a M211 but single rear wheels and a correspondingly narrower cargo bed with wheelhouses instead of a flat floor. They were also 100% equipped with hard roofs (snow).
The electrical system of the M135/M211 series was 24 volt. I believe they were originally built with the metal Douglas connectors but I have seen some parts modified (in different levels of quality workmanship) to use the rubber Packard connectors as the supply of switches, gauges etc with Douglas connectors dried up. You may be surprised how fast the cost of little parts add up - don't throw away any connector parts that could be re-used if they look in tolerable condition (the ugly but functional can be used in places that aren't easily seen).
The military manual for these trucks is TM 9-8024. Original paper copies are frequently available on ebay. Today, the prices start at $12.99 plus shipping. Reproductions can be bought from
https://www.portrayalpress.com/searchresults.asp?cat=1830 Or if you prefer to download it can be found at
http://www.jatonkam35s.com/jatonkaM135-211sTMdownloadpage.htm there may be other sources too.
My recollection is that most circuits were 14 ga but you should check this rather than trusting my aging memory. You may also be able to read the labels on the existing wire.
I need to build myself a trailer wiring harness and was looking for sources of wire appropriate to military trailers. A source found online is
https://www.eriksmilitarysurplus.com/vehicle-electrical-parts.html but I have no experience of dealing with them.
