My Vanagon has the dreaded fast idle problem 2200 rpm, but a blip on the
throttle to 4000 will bring it back to a normal idle. I was looking at the
shop manual intent on tracing the problem, and it makes reference to a
special wiring adapter needed to make some of the tests. Can I make one of
these? Or do I have to buy one from the dealer?

I pulled the relay and cleaned the contacts which made things a little better
- but not for long. There did not seem to be any burned components in the
relay - and the idle regulator valve appears to be buzzing.