goto the following link for the Factory Service Manual for your vehicle:
http://www.nicoclub.com/FSM/quest/1999_Quest/
should have acrobat pdf reader ready...
goto "ha" section (heating-air conditioning)
find "heater core" on page HA-211
------------------------------------------------------------------------------
wow, nice that you don't have to discharge the AC refrigerant to R/R this unit!
edit: well if you have the Automatic unit, there's only 1 additional step & it's on page HA-119