[Landseer]

Clean the ground under the engine.
While there, inspect/clean the starter electrical connections.
Look first for voltage there, then disconnect battery and clean them.
The 14 pin connector --- did you clean that, it channels power to the solenoid on the starter.

Could be the starter motor, perhaps light hammer taps and try it.
Could ignition switch electrical portion, also a common failure area.

Maybe Sean or others will provide more definitive ideas or tests?

Key thing is grounds, electrics and the right replacement relays (gotta be precise on early cars) as a firm basis from which to begin.