92 Mustang won't heat up.

There is no other reason the water temp wont come up other than the thermistat.

Whether the gauge moves or not, start the car run it for 10 min, then feel the upper hose and see how warm it feels. If that is warm your gauge may not be working properly.

What compressor, your AC??? Why is that on?? Do you have the defroster running?? how is the temp of the air out of the heater??

thermostat

had the same problem with mine and it was the thermostat. I changed it and the stupid thing failed almost immediately. finally bought the cheapest oem style one and it works great. change it out again until you get one that works...

its a common problem with 4bangers, i wouldnt worry about it. 9 times out of 10 its just your gauge giving you a bad reading.