Sorry, it's impossible to replace without dash removal. You have to get into the heater/AC box. Take your time and tag/bag every screw and bolt. It's really not too hard. Just tedious.
Thanks everyone who posted solutions to bypass my heater core at work yesterday! Made it home AOK... Now, the ugly task of replacing it with one that doesnt leak! My question is... Is taking the dash out the only reasonable way to swap out the heater core with a good one? Anyone ever left the dash in and done it? All tips/suggestions are appreciated! Thanks everyone.