For over a year, maybe every fourth time I've driven my 90, the
coolant light would flash and keep flashing until I popped the
instrument cover, reached behind the gauge, and pressed the foil
wiring. Sometimes I would get a mild shock that I presumed was from
the capacitor discharging, but I'm not sure.

Anyway, I got so good at it that I could perform this ritual while
driving, to the horror of my passengers, especially when I handed one
of them the plastic cover to hold (they were concerned that I was
"tearing the dash apart." but I got it down to about two smooth
motions).

About a month ago my odometer stopped working. I pulled the
instruments and slid the gear back over the worm gear. While it was
out, I took a small wrench and tightened the several brass nuts on
the foil.

The inappropriate flashing stopped for good. It only flashes now when
it should.

It may not work for everyone, but the opportunity cost of this fix is
pretty low if the cluster is already out.

Jim