i did thi change recently. Its not a fun job but it can be done failry quickly.
Remove all dashboard including metal braces either side of the centre console. (30-40minutes)
Loosen all of the bolts holding the blower, aircon, and heater system. (10-15minutes)
Wiggle (yes wiggle) the heater system away from the aircon - so you dont have to degas it - it goes towards the drivers side (40minutes + possibly a broken plastic tab or 2)
Replace old core with new (2minutes)
Put back together (60minutes + bandage fingers)
Reconnect hoses and thank all forms of religous icons that you have heating again
