This depends on what you are calling heater cable. Most heat cable has two segments the heat tape and the cold wire. These are usually spliced together in either a high-temperature box or as an in line splice.
The heat tape is the portion that actually gets hot and should not touch the plastic housing of the SST-2. The cold wire is the normal insulated interconnect wire used to provide current to the heat tape and can enter the SST-2 box to connect to the output wires.