I found many answers regarding intermittent starting, but answers basically cover the entire starter system and I'd like to find a more specific answer to my problem.
1999 Toyota Tacoma 4x
Intermittent starting trouble with new battery
This answer seemed closest to answer the specific issue I'm having and suggests that the intermittent starting could be the starter itself.
This problem started for me as occasional and only when the air temperature was very, very cold. I turn the key, hear a click, starter doesn't engage. Everything else works fine.
I thought it might be the clutch bypass switch because when I pressed it, the engine started next time. This turned out to be a coincidence because as the frequency of failed starts has increased, and now happens every time regardless of temp, that trick no longer worked.
I got a new battery and replaced the terminals. No effect.
Now it fails about 90% of the time and sometimes the starter engages briefly and then stops.
So I'm thinking starter based on the explanation in the referenced answer, and not solenoid or switches. Thoughts?