Without seeing, I'm betting its an AIC (aux. idle control). It was an option mainly used to adjust the idle for quicker warm up-or to run a pto. Best thing about it is its also a digital tach. Mine is mounted in the passenger side kick panel so it must have a fairly long harness. I've seen pics of them mounted on the doghouse or dash.
If you decide you don't want it, sell it on a diesel forum-they go for a couple hundred.