as detailed by ray (rad4runner), the wiring flaw is that for those trucks that were originally equipped with a starter relay (not all 1st gens or pickups were), the factory wiring was wrong. the way the wiring was done didn't utilize the very reason a relay is used in the first place - not feeding the full load through the dash/switch stalk.

the way it was wired meant toyota wasted resources on installing the starter relay, it didn't do anything useful relative to the trucks that didn't have a starter relay.

 

I raised the pickup onto jack stands. Very solid with room to roll over.
My main source of instructions for removing the starter was: The lower bolt was tough. I loosened it with a 12 inch breaker bar on 6 inch extension.
edit I unscrewed the clutch slave cylinder and tied it out of the way to increase access to the lower bolt. The fluid line was obstructing the socket.
My neighbor helped me to remove the upper nut. Actually, the nut was frozen to the stud. They came out together.
edit. I use an oil filter that is about 2" longer than factory. I drained the oil and removed the filter to to make a straight shot for the extended socket wrench (about 28" from wrench to end of socket, including a U-joint next to the socket)
The starter fell off the engine over night and came out easier than my alternator ever did.
The rebuild of the solenoid was straight forward. No broken screws or knuckles.
I need to bench test it. looking. edit. That weren't so hard. The starter works well,
Pic attached of the old solenoid and contacts. The lower contact was pounded about 0.040".
Time to reassemble.

 

the plunger is a replaceable part, too. that outer edge (a "ring) can get thin, just like the one contact you replaced. i've replaced contacts in three starters: both '87 1st gens, and my former '97 corolla. corolla had the same starter/solenoid contacts. so did my '89 camry, but i never had to replace those.