I hope someone answers your question, my neighbor has a 96 E 350 motorhome chassis that does the same thing. I spent about 3 hours working on it last fall, and all I got determined was the wiring from the driver area to the rear of the vehicle was okay. without a schematic I wasn't going to start chasing wiring up under the dash.
I have a suspicion it may be the turn signal switch, but I've never had time to get back to it.
It has another thing that I never chased down either. Plugging my scanner into the OBD II system shows nothing, no readings whatsoever. Of course it's a neighbor, so he expects me to do it for free, my time is too valuable to waste a few days chasing electrical problems on some 25 YO motorhome.
Last I knew it wasn't starting, so maybe he'll give up on it, his health is declining due to cancer, and he told me he's not well enough to go camping any longer.
I like the guy, but he has zero mechanical aptitude, and buys old cars that have problems that aren't easy fixes. The place looks like a used car lot, and maybe 2 of the 15 vehicles he has, run, and are roadworthy. The rest just sit in the yard. I think when I bought my house, and he found out I was a retired mechanic, he figured I was going to get all his junk running, I have plenty of work to do on my own old junk to keep it on the road......
