Do the interior lights come on with the headlamp switch? Have you checked the fuse for the interior lights? Have you checked the bulbs? How many lights do you have? One under dash, two under dash, and/or back top of the cab?
If you're not much of a night driver, then why even do the effort of adding relays? There are sealed beams that are a cleaner white for better performance.
I can't see the boards cut out being much different from a 72 to a 73 and up truck. The width and length might be a little different but the cutout for the clamp shouldn't be.
https://gmcpauls.com/47-72_BedWood_Info.htm
Generally, cables don't corrode within a year. As @82sbshortbed stated about checking grounds on the frame or engine. Any paint, primer, rust, etc. will degrade the path for voltage to flow. Clean metal, washers, bolts, and terminals are key.