The car is a 1980 245DL B21F/M46. Originally would crank fine but wouldn't start. Jumped fuses 5 and 7 (in-tank and main fuel pumps, thought it was the fuel pump relay) momentarily, car started and ran. Then that trick didn't work the next time, eventually found the negative battery cable was disintegrated, replaced it, recharged battery, then would crank great but not start, jumped fuses 5 and 7, car started and ran great. Re-started a few times without a problem (didn't need to jump fuses), left it overnight, and now it won't crank, won't even try to crank, no starter click, there's no power at all - the clock is barely ticking and the accessories don't work regardless of ignition position. However, my meter reads 12.5 volts across the battery terminals and at the positive cables leading into the wire harness that runs into the vehicle. There appears to be no power at the fusebox in any ignition position, unlike before.

Do I check voltage at the ignition switch next and how would I do it ideally without removing the switch? If the switch is bad would it prevent power from getting to the fusebox? Anything else to check? Is there an easy continuity check for the harness into the fusebox? The ignition switch was replaced at ~190k miles, car now has 286k.

Thanks for any help, this car runs too well to be killed by an electrical gremlin.

Zack
1980 245DL M46 286k
1988 745T+ M46 223k