I think I need a new heater core. I have no heat, and the windshield fogs up instantly. My Regal is an 87 (RWD) with A/C, so my question is this: to replace the heater core, do I go thru/under the dash, or thru the engine bay where the hoses attach to the core?

You go through the engine bay. You have to remove the vent cover first. After about 30 screws later, you'll get to the heater core. It's not hard at all but does take some time. Keep track of where the screws go.
Good luck.
MARK