My diagram shows that R1 andR2 on the key switch (bottom 2 terminals ?) are tied together ,and then a single wire goes from there to the thermo. The picture shows that the wire to the temp.sending unit,up on the thermostat housing,and the thermostart wire,are together in a wiring harness-just those 2 wires.So follow the temp.sending wire down,and you should find the broken wire. You shouldn't ever need to use thermostart,unless it gets down below freezing.