temp sender unit 357919501A, this has been superseded but if you google the part number you're sure to find something.
your temp gauge not reading can only be down to three things:
thermostat not closing, then heating will be poorer (but usually still work somewhat)
temp.sender unit -this is a dual sender unit, one going to ECU the second the instruments
electrical issue (wiring or instrument cluster)
check my previous posts here. there is an image that would allow you to measure the sensor!