OK. Thanks for dialing it in. Turns out, I did see both
the square and round doohickeys in my initial under the
hood gropings.

Rapped both a couple of times with a screwdriver handle.

Weird thing. Drove 200 miles this afternoon and evening.
About 30 miles into the trip, the tach started working.
Worked all the way to my destination. It was nice to see
how many rpm's I was turning while running 70.

However, after dinner and a movie, the darn thing
persistently refused to work again on the trip home.

I can't have this. Cannot allow a device to tease me
with intermittant functioning. Will do some fooling
around with this thing tomorrow.

Update will follow.

Appreciate the help.