On the Fiat 124 series the heater valve is located under the dashboard on the passenger side. Due to the valve design a failure usually results in leakage or a complete failure where coolant is dumped all over the interior.
When it was time to replace the valve on my 124 I decided to try something different. I had picked up a heater valve from a 91 Honda Del Sol at the wreckers. Using the remnants of the old heater valve and a 90 degree bit of heater tubing I was able to install the Honda valve outside of the interior. The fit is not too bad plus this valve appears to be much more functional than the standard Fiat valve and will probably outlast the car itself.