There are a hundred answers for that question on here. However, the easiest way is to remove the passenger side fender to gain access.
remove the entire dashboard.
heater core is shot...
factory recommendation is to cut a hole in the inner fender well to access/replace it.
you usually have to take apart the entire dash to get to it
where is the heater core at
where is the heater core at
u need an 8th inch enkay heat shrink heater hose and some clamps
where is the heater motor located at
heater core has a leak replace heater core
It is cheaper to replace the heater core than it is repair it
You can replace the sheer core, on your 2004 Pontiac Grand Prix, by removing the water lines. Remove the four bolts that hold the heater core in place. Reverse the process to install the new heater core.
It's tough for an amateur. The heater core is inside behind the console.