Re: 1970 Mercury Montego electrical problem
Take 15 minutes and remove the starter. Use jumper cables from battery negative to the body of starter. Put positive cable on battery and with starter on the ground, press the positive jumper cable onto the copper terminal on side of the starter. If starter is ok, it should spin and jump. If it's sluggish, maybe it's near the end. If the starter is ok, you can spend time on all the electrical path between battery, ignition, and relay.
1971 Torino GT, 351C-4V, automatic, AC, 4.11 rearend, holley 750.