Assuming this is your 87 V20, since you didn't say, probe both sides of the 5-amp instrument lamp fuse using an incandescent test light with the running lamps switched on and the dimmer rheostat rotated fully CCW. If only one side of the fuse is hot, replace the fuse. If there is no power to the fuse, check the headlamp switch connections and/or replace the switch. If both sides of the fuse are hot, check the instrument cluster ground connections to the cab sheetmetal and the harness connections to the back of the instrument cluster; replace any bulbs that are burned out. There are other possibilities, but this will get you started.