If it's a 90, I just went through finding that thermo switch myself...

It's under the intake manifold, next to the oil filter.

My various manitenance manuals either say it's located at the bottom of
the radiator or on the thermostat housing. On a 90 Civic EX, (pretty
sure CRX is different) it's at neither place, but under the intake
manifold next to the oil filter... the only way to access it as far as
I know is from beneath. Need to wrap new switch threads with teflon
tape per the Helm manual. I got mine at Napa and it came with new
o-ring.

Good luck.