The sensor wires are connected to the blue #18 awg wires in the low voltage section. The sensor is not polarized so it does not matter which sensor wire goes to which blue wire.
A representative diagram of a temperature sensor for an SST–2 system installation along the length of a pipe is shown above. Note that the heat cable and temperature probe do not cross.
If the pipe is insulated, the insulation should wrap around the pipe, as well as the temperature probe and heat cable.