Won't crank

I had exactly what your describing last week in my 71 Duster. I started it by jumping the relay and got it back in the garage. I had power to the yellow wire terminal on the relay, but the relay wasn't "relaying" the power. Took the relay out, opened it up using snippets to fold back the tabs and remove the back. The windings were in good shape, but there was corrosion. I cleaned all contacts real good, folded the tabs back with a small punch/hammer, reinstalled, and it's started about a dozen times since. Plan to get a spare relay just in case...but my thought is the old stuff is built better than the new so I'd rather clean it up and press on. 67Dart273 has forgotten more about Mopar mx then I know...so follow his advice and you'll fix your problem and learn something!