Don't know if the 99 has this function or not, my 01 does, there is a way to test the cluster though I cant remember how it is done. Hold your left ear with your right hand, stand on one leg, press a button turn the key. Then it will illuminate all the cluster lights and sweep the gauges that way you will know if the light is out or something else. I had an ecm that was wonky and my wait to start light would not come on unless I reset the ecm (though my grid heaters would not work either at those times) then it would work for a while then after a week or so stop coming on again a refurb'd ecm fixed it.
I just did a quick search and found this on cumminsforum not sure if this it but it sounds like it was what I did now that I see it though it has been 10 years, don't have my truck with me so cant verify it.
"Testing the instrument cluster: (Start with the key in the OFF position.)
-Push and hold the trip/reset button on the instrument cluster.
-Turn the key to RUN (Do not start the engine.) while you are still holding the button.
-The word "CHEC" will appear in the odometer display, you can let go of the button now.
-Note down any numeric codes that show up on the odometer display. They will help in troubleshooting."