I googled M38A1Cdn2 wiring harness and also M37B1 turn signals, and both searches will bring up your turn signal system. The Cdn 2 wiring diagram is actually on this site. Below I have attached a photo shamelessly stolen from the internet which shows the relay box you are asking about.
There are the solid state dignal kits which replaced these earlier boxes, but since your truck is rather unique, I would think you would want to keep the original set-up. I do recall seeing the installation kits available at one of the US suppliers like Saturn surplus, but I can't find the source now.
Found it. It was Saturn surplus who has the
NSN: 2590-00-906-0155 boxes listed:
http://saturnsurplus.com/electrical/0155.htm