Tref, I believe your temp. sensor goes to the glow plug relay, and then to the gauge. See if all your connections are in order. The resistance of the temp. sensor is:

At coolant temperature:

0 Deg. Cel. is 8200 Ohm
+25 Deg. Cel. is 2440 Ohm
+80 Deg. Cel. is 290 Ohm

If two gauges did not work, it is most likely a loose connection somewhere. Good Luck