suzuki removed the Tstat and hot wired the radiator fan - shoot them, do not let them work on your car, find a good workshop. and if its still heating up with no tstat and direct fan verify that your coolant pump is actually moving coolant. If it is and still not rectifying, check the radiator fins for blockage. Still not working, check compression timing on the timing belt, correct, do a leak down test. If all are good.
Check the gauge itself, find another mehran and test the resistance of the sender to ground when the gauge reads half, check your thermistor and to check the gauge also buy a resistor of the same impedance as you checked the other cars resistance to be - stick one end in the wiring and one end to the engine block, it should show half needle if the gauge is correct.