Look here (post #10) for a list of cluster lights that are driven by the PCM bulb-check/self-test. JimDuchek on CumminsForum has posted about that a number of times - he seems to be pretty smart on the electrical side of things.

If you don't already have a factory service manual (FSM), you can download one for free here or get a paper copy at Geno's Garage - the paper copy may be easier to use if you have to trace wiring diagrams across pages.

Good luck finding your problem!