Steve
I have 1997 Yorktown and I'm not sure wheather or not the evaporator box and front cap are the same but on mine there is a removable panel in the front cap above the generator slide that gives access to the evaporator box .
To replace the heater core or evaporator you remove the front side of the evaporator box and is right there.
But as I said I'm not sure it's the same on the 2004 model year.
Eric