Recently I replaced the heater control switch, but did not put that little coiled tube thing in with the core, as the old one was situated. The heater works fine (the "old" one sprung a leak) so can someone please let me know what that little tube is called and what it's funcion is?
Thanks.

Pearl;

That capillary tube was part of a "closed loop control" system ala 1960! The bulb at the end of that tube was located to sense the temp of air out of the heater system and it fed back to the valve to modulate same and keep a set temperature.

If your replacement valve works well and doesn't leak, that's great...the automatic regulating part is not as critical in my opinion...I can just adjust the control manually.

Cheers!