Seeing stuff like that makes me so glad I'm no longer in service engineering at Ford Motor Company. The technicians would contact us about stuff like that ALL the time and we had absolutely no reference because it was added to a cutaway chassis by the bodybuilder. The tech was kind of stuck because he was supposed to fix it (and sometimes even tried to claim warranty time
) but had nothing to go on.
Upfitters are really good at doing things like using black wire...and only black wire...for EVERYTHING. Oh the fuming customers...the A/C wouldn't work (performance is lackluster on a stock E-series due to airflow restrictions)...come to find out that the aftermarket upfitter installed a second evaporator unit and tied it into the stock system without upgrading the compressor or condenser. Upfitters would by an E-250 to upfit because it was cheaper and the customer would get upset when Ford wouldn't warranty the broken suspension springs because it was 50% over the GAWR before the customer even added their own junk to it.
Speaking of Ford...there is some information out there that may be of some use to you and it is absolutely free. There is some good stuff at https://www.fleet.ford.com/truckbbas/
though a lot of it applies to newer models only. Of course, the E-series really hasn't changed all that much...