Jon, I got my harness for my 56 3100 from Lectric Limited and i am very happy with it. the quality is good, it fits perfectly, etc. The installation wasn't "trivial", with some color code confusion along the way, but in the end i was pleased with the product and the outcome. My truck is in Seabrook NH if you ever get the notion to take a look
http://www.lectriclimited.com/tom