Every Ford chassis larger than a passenger car I've dealt with has had a plug under the dash. It's always grey on Fords. There's only two types Ford uses. The F53 chassis uses the old style and is to the left of the column. Your E350 chassis plug should be just to the right of the steering column. The F53's come with the adaptor plug with the manuals but I'm not sure on the E350's.
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