The Central Oklahoma Classic Chevy Club has online assembly manuals for the 55-57 passenger cars. You have to go to their main page and choose reference section, then 1955-57 Assembly manuals, pick the manual you need and then scroll down to the section on Transmission Overdrive. You'll find diagrams of the control, the harness and relay, a wiring diagram and the kickdown switch for both the 6cyl and 8 cyl engines.
I'm not sure how much this differs from the trucks but it may help.
http://www.55-57chevys.com/coccc/index2.html
