in response to your problem, the best way to tell if your headgasket it gone is when the engine is running, look out the tailpipe and look and see if you have white smoke coming out of your tailpipe. If it smells sweet and white smoke is accompanied by it, your head gasket(s) may be gone. To replace the heater core is a tedious task. You have to remove the dashboard just to get to it. The exact steps to do it (i've done it many times, I'm a Chrysler tech) discharge your a/c, drain your coolant, remove the heater hoses and a/c hoses from your firewall, remove your steering column, remove dash. It's hard to do if you're inexperienced so I recommend taking it to someone you trust and know. If you want to tackle it on yourself, be patient, don't rush, pay careful attention to every little detail, bolt, electrical connector, etc.