IF your truck didn't come with a trip computer, the wiring cable that runs from the fuse block under the driver's side of the dash up to the overhead console probably has only two wires in it, for the overhead lights and the high-mounted stop light, I believe. To run the overhead console, a new, more complex wiring harness must be used, as well as a smaller interconnect cable up top.
With the much appreciated assistance of a local diesel guru, and at great personal risk to mind and body, I was able to ferret the numbers off a 2001 truck on an undisclosed local car lot. My chum was able to source me these part numbers for my 99 QC. There is no reason to believe that they would have different part numbers on later trucks. Use these numbers at your own peril.
Main Harness: 56045921AD
Upper Harness: 05013608AA
I would think that used trip computer modules with all functions working would be a precious commodity, with an equally precious price, with a 5-second or 5-foot warranty. I's gonna get a new one from the dealer, as soon as my plastic has stopped smoking from the last round of... . um, excess.
Good luck.
Tom