Forget that other stuff for now. It's Battery, Starter and three wires:

1) The big red cable from Battery Positive to the starter (fastened with a nut to the threaded post on the solenoid—the small "can" on top of the starter.

2) +12 volts wired to solenoid terminal 50 (Small flat terminal below and to the outside of the big post on the solenoid,

3) The big Blue connecting the Battery Negative to the engine and/or chassis metal, which is the "Ground". Nothing works without it.

Disconnect the Blue Ground cable at the Battery first (always Blue first), and then disconnect the red cable to scrape/sand/file both end terminals (and the Battery Positive Post) shiny clean.

Do the same for the blue wire and its Battery post—and the other end at the car/engine "Ground" points.

Then try it and let us know what happens.
--
Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Sold 5/03)
'83 Turbo 245
'76 244 (lasted 255,000 miles)
73 142 (98K)
'71 144 (track modified--crusher bound)
New 144 from '67 to '78
Used '62 122 from '63 to '67