Heater problem
1991 Holden Jackaroo 4 cyl Four Wheel Drive Manual 110000 miles

Hi, My heater in my 91 holden jackaroo is not working, it blows cold air.
I thought it might be the thermostat so i replaced that, also had to flush out the radiator with a radiator cleaner because it water was a rusty color. Replaced with coolant.

There is no coolant inside the car so i don't think it is a cracked/leaking heater core.
But could it be blocked?

Also the temp gauge is not working, im trying to diagnose why not at the moment.
Would the heater not work beacuse the temp gauge is not working???