I have a 1967 289 Mustang with factory A/C. I hooked up the heater lines and she leaks like crazy so I'm guessing the core is shot. Difficulty in replacing this? Also when running A/C only could get air flow out of lower vents. Not uppers so again car is 50+ years old the plastic flaps are more likely broken. Difficult, entire dash has to come off I guess?? Let me know....thanks