Water icon on dash is glowing (not bright and full on) but just glowing this is new?

1992 300E 112,000 mi. The water (anti-freeze) level is to the line in the well in the engine. Just had a new thermostst installed. Temperature indicator is normal...slight...slight change upward with new thermostat...from 80 to 90. Sensor cacked but this happened a long time ago (2 months) and it worked perfectly since. So....could it be that the crack in the sensor at the side of the well has accumulated some debris and it is now blocking contact...would this show a slight...slight glow? BTW can only really see the glow at night...have to look for it in the daytime...in the sun. About to drive it to San Francisco from LA next week...temp could climb to 90 on Hwy i5 as I drive. Phew! Frank