I have a wire cable (small) that goes from control panel to lowest pipe in the tub. This pipe is a main line going to the pump. Is it possible that it is for a sensor to keep the heater from coming online when there is no water in the tub? I'm hoping this to be the case. If not, what control the contactor for the heater?

It's not a wire but a capilary tube for the thermostat (DO NOT cut it, if you do it's not repairiable and you'll have to replace the thermostat).

What prevents the heater from coming on without water in the tub is ether a flow switch (that senses water flowing through the system), or a pressure switch (that senses the increase of pressure in the pipes when the pump is pumping water).

Thanks Doc, didn't think about that!!!!!!