I am in the process of removing the engine from my 96. I have noted a rigid thermal copper wire, that goes from the heater valve control in to the heater matrix box.
It is certainly not for adjusting the water flow to regulate the temperature. as I can see that cable quite clearly.
It's a thermostat, there's a coil on the end but I can't remember where that goes exactly. The wire is filled with ether, so make sure you don't damage it (although I'm sure you can happily live without it).
I believe that the phile terminates in the warm air outlet duct/box (inside the car). It helps maintain a constant temperature outlet. Must admit that I can't see the reason for it except the designers thought it would give a more refined/accurate control of temperature.
when I took my heater box apart it was just coiled up under 2 metal tabs inside the heater box (probably just to stop it rattling) I replaced my heater valve recently with an E-Type valve (half the price) and just pushed the coiled wire into the hole