I went through this agony with my 89 and found that when the starter
wouldn't spin, I could start it by shorting the starter terminal with a screwdriver. Then the starter might work for weeks or months before it quit again.
I tried another starter from the boneyard with the same results.
I did the battery cables and interlock etc with no luck.
The car went to the junkie with that unsolved problem

Now I'd try a NEW solenoid and wonder about the solenoid lug being
just riveted on and maybe with corrosion can act like a switch?
BTW there's a pair of spade lugs for that solenoid wire,an upper and lower.
Maybe one will work better?

To get that top starter bolt out, I found that you need a stubby 3/8 drive
swivel socket and a long extension/s, maybe 3' and it comes out easily.
It jams up with ordinary sockets with not enough room between the bolt head and the firewall.

Good luck, Bill