2003 Mitsubishi Eclipse Heater takes forever to get hot

Go get a prestone flush kit, then install the tee on your heater line, open the drain at the bottom of the radiator, this may require the removal of the splash shiel under the engine, Attach a garden hose with the supplied adapter, turn it on after the old coolant is drained, run till water is clear. Then drain the water, close radiator, fill with 50-50 antifreeze mix!
To get a good flush of the heater core clamp off the hose behind the tee, this will increase flow thru the core. Change the thermostat.