The Temp Sensors (Thermistors) are on the heater. They are not interchangeable, one is for the control thermostat system, the other for the High Temp safety system ("Hi Limit"). They cannot be installed in the wrong spots as they have different size threads and different connectors.
The place to feed in the snake would be the fitting nearest the wall of the motor compartment - you will have to remove the vinyl tubing which goes from the heater to the wall. With the tub empty, you may still get a small amount of water out of the tubing, but slide the clamp back and work the tubing off the PVC barbed fitting which leads into the wall of the compartment. It may have a short length of flexible PVC showing, that's the very "pipe" you want to clean. You can also get to it from the drain fitting in the bottom of the tub, but then the crud has no place to go. It needs to have some water flow to work the stuff out of the tube as you break it free.
You will have to remove the circ pump to clean it - I have flushed them out in-place by removing one tube and letting the water flow out as I carefully worked a wire or other probe into the pump, but the best way to get it really clean and clear (by far) is to remove it and dismantle it.
HTH