I bought a 2003 chey duramax the dash light keeps on going out have to hit the dash for it to work

loose bulb?

sounds like loose fuse, loose connector or loose ground - especially if a whack corrects it. Check all your fuses tight in the top row of the fuse block at the driver's end of the dash - there's one there near the middle labeled on the cover as "illum" which feeds the dash lights - you may even want to swap it out with a fresh fuse. The BCM handles all lighting circuits and is under the dash just to the right of the steering wheel, pull off he instrument bezel (just pull it towards you - it'll snap right off) to expose the BCM and wiggle and press in on all the connectors. Take a flashlight and check on your visible ground wires around the instrument cluster and up under the dash on the driver's side- may need to tighten or re-connector and re-attach one.