73 duster wont crank

its an automatic 904 torqueflite with a 225 slant 6. my battery cables are connected good and my battery isn't that only just like 6 months but its like its not giving the starter power. ill check my starter relay and ill look under the hood for the seat belt interlock but i don't think i have one or it didn't affect my ignition because the retractable part of my seat belts weren't hooked up and it wasn't phased and does the body of the starter relay ground it? cause the bolts that hold my starter relay is don't go in far enough so it has wiggle rook