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You have a bad heater core. It's a fairly hard job to change.
Here are the instructions
REMOVAL & INSTALLATION
See Figures 1 through 6
Drain the cooling system.
Remove both heater hoses.
Remove the lower right hush panel.
Remove the lower right instrument panel and the ESC module if necessary.
Remove the lower right instrument panel-to-cowl screw.
Remove the heater case screws.
The upper left screw may be reached with a long socket extension. Carefully lift the lower right corner of the instrument panel to align the extension.
Remove the case cover.
Remove the support plate and baffle screws.
Remove the heater core and baffle plate from the housing.
Position the heater core and baffle plate into the housing.
Install the support plate and baffle screws.
Install the case cover.
Install the heater case screws.
Install the lower right instrument panel-to-cowl screw.
Install the lower right instrument panel and the ESC module if necessary.
Install the lower right hush panel.
Install both heater hoses.
Fill the cooling system and check for leaks.
Fig. 1: Heater module assembly
Fig. 2: Removing the heater core inlet hoses
Fig. 3: Removing the sound insulator attaching screws
Fig. 4: Once the sound insulator has been removed, the heater module can be accessed
Fig. 5: Removing the heater module cover
Fig. 6: Be careful not to allow any coolant to drain on the carpet when removing the heater core
Yes, Jon is correct. Bypassing is an option but it's going to get pretty cold and you won't be able to defrost the windshield.