The temperature gauge in the dashboard of my '77 Toyota pickup stopped working after I flushed the cooling system and replaced the thermostat. The temp needle doesn't move. The motor had been nearly overheating; the radiator was very rusty, and in the process of flushing, de-rerusting, and cleaning the cooling system I noticed the temp gauge was fluxuating a lot. Water got splashed around under the hood, which might have shorted something? But I checked all the fuses. (Should I l ook for the temp sensor somewhere -- where? Could the gauge itself go bad suddenly? Or, what else???)

Thank you!