Once you get to that point it's not much work to remove the rest of the dash and replace the core the proper way. If you're not replacing the AC evaporator core, then you don't need to disconnect the lines through the firewall. You should be able to just pull the HVAC box back a couple inches in order to replace just the core.
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It's been a loooong time since I've taken a dash apart to fix a heater core (knock on wood).
But one thing comes to mind; when a vehicle is built the entire dash assembly is pre-assembled, radio, wiring, gauges, everything. They just slide the whole assembly in with a long arm and put in a couple bolts, hook up the wires and away it goes. Only takes 30 seconds or so and it's done.
Haven't put a whole lot of brain power into how you could pull the entire dash back out that way. Just thinking.
Ive THANKFULLY.... never had to do a heater core. But Im all for opening 'er up to get more access. In his case Its all going to be covered afterwards. Im not sure how essential it was to cut, but if it helped get to some bolt heads or something then hell, id probably do the same. Ive changed many a fuel pump by cutting holes in the trunk as opposed to dropping the tank.
I just replaced the heater core in a 5.9 ZJ and there's no way I'd suggest cutting on the dash or anything other than the coolant hoses, if you're gojng to be replacing them... There's two coolant hoses, around twenty screws, a few bolts and ten or so electrical connectors to disconnect, then the dash has room to be pulled away from the firewall enough to remove the entire HVAC assembly, if needed... The entire process took me about 6 hours, and there was nothing difficult about it... I think the most difficult thing about it was getting the coolant return hose off the old brass heater core outlet... lol