Tapping into thermistor output
Typically a thermistor is biased with a low constant current supply and the resulting voltage across it is measured to determine the temperature. If you select an op amp or instrumentation amp with sufficiently high input impedance (e.g. > 1 megohm) you can use this to measure the voltage across the thermistor without disturbing the incumbent ECU electronics. Due to the high input impedance, you should consider incorporating a low pass filter to reduce noise pick up.

The new op amp circuit can then drive an analog voltmeter that can be calibrated with a temperature scale. You can determine the op amp gain, offset, and scaling factor empirically if you do not have access to the NTC specs and knowledge of its bias current.