thermoswitch to run radiator fan

"... please suggest me .... if there is any type of sensor/switch(combined RTD and thermistor) which wil show continuity even under "off" condition? ..."

Such a device is commonly called a piece of wire .

I am not sure I understand why you want to change a system that has been working so well. But I would like to offer some ideas for you to think about. In the cars that use the system you describe, they add an additional circuit to turn the cooling fan on whenever the air conditioning compressor is turned on. This would allow the back up to the thermal sensor you have now, with no additional switch needed. Another way to achieve what you ask, is to find the fan temp sensor, and run an item described in post #1 to a switch installed within driver's reach. If the sensor has two terminals, you will need two of the items from post #1. Good luck.

About the only way that it could be set up is to use a separate latching relay and wire the relay in parallel to your existing thermostatic switch with a different temperture sensing switch that would be set at a temperture that is higher than what is now activating the fan and once it latched in it would not release until the power to the relay was broken. If you did set up a system such as this then you would also need to have a indicator that would show that the system has been activated so that you would know that you needed to repair the primary system. Hope this helps

If you place a second thermo switch in parallel to the first one and one fails in the open condition, the other will still control the fan correctly.

Check from time to time to make sure that both are still in a working condition using an Ohmeter.

Its probably a good idea to also have a remote thermometer that you can read to allow you to check on the actual temperature from time to time..... a normal switch also wired in parallel to the thermo switches will allow you to switch the fan on any time you wish!!

Good idea. The second switch should turn on a few degrees higher so that it never operate unless the first switch fails open. This way you double the life time of the system.