Getting a wire through a 2021 1500 Silverado LTZ Diesel requires removing the very heavy battery which necessitates removing the fuse plate from top of battery, be patient and take pictures. First remove ground cable and associated wires and insulate terminal, be careful not to touch battery terminal a second time or you will set of alarm. Remove the 6 cables and 2 clips from fuse holder plate, they are all marked and numbered, take picture to be safe. The plastic fuse holder has 2 clips on engine side that require inserting thin flat blade down to unhook, also one clip on fender side. Loosen positive battery post and finagle housing up till it comes off battery post. Tie all connector crack to air conditioner line. Undo 1 battery clamp and remove plastic lock plate. You will need to tip battery on an angle to remove, it’s heave so lay a blanket or pad to protect plastic and paint. Behind battery on firewall you’ll see a boot with a small nipple boot beside it, cut a slit in end. I uses a thin fish tape and pushed it thru, there is a lot of stuff back there and tape will most likely hit something. Take the felt panel off passenger side and look up with a light and you’ll see your fish tool, it was too tight to get my hand in so I used a coat hanger to snag fish tape while wife pushed from under hood. I was able to tie a string and tape my wire to fish tape. Pulled fish tape out, which pulled back wire and a string if I ever need more wire. Hope this helps someone else.
