Heater will not get warm

First, the idea of the gauge moving that much makes me believe there is an air block in the system. Make sure the coolant is full and see if you can bleed the system. Loosten the return line from the heater core to see if you can get air out. If that doesn't help, start the engine, turn the heat on high, and let the engine warm up. Both heater core hoses should be hot. If they are, check the blend door under the dash near the heater core for operation. If only one is hot, try flushing the heater core.