Here are directions I was able to get for you and yes you need to drain the coolant.
1. Disconnect the negative battery cable. Allow the engine to cool down. Drain the cooling system to a level below the heater core fittings on the firewall.
When draining the coolant, keep in mind that cats and dogs are attracted by the ethylene glycol antifreeze, and are quite likely to drink any that is left in an uncovered container or in puddles on the ground. This will prove fatal in sufficient quantity. Always drain the coolant into a sealable container. Coolant should be reused unless it is contaminated or several years old.
2. Disconnect the heater hoses from the heater core tubes and plug hoses.
3. In the passenger compartment, remove the four screws attaching the heater core access cover to the plenum assembly and remove the access cover.
4. Pull the heater core rearward and down, removing it from the plenum assembly.
1. Position the heater core and seal in the plenum assembly.
2. Install the heater core access cover to the plenum assembly and secure with four screws.
3. Install the heater hoses to the heater core tubes at the dash panel in the engine compartment. Do not over-tighten hose clamps.
4. Check the coolant level and add coolant as required. Connect the negative battery cable.
5. Start the engine and check the system for coolant leaks.
Let me know if this helps.
Wednesday, August 4th, 2010 AT 7:14 PM