Where is the heater core on a 1994 Grand Prix?
Heater cores are located behind the dash panel in a big black plastic box (passenger side of the car). The water comes in through the firewall of the car and circulates through the core inside the car and then flows back out to the engine. If you need to change it...my prayers are with you. I have done this before on other cars and pulling the dash panel is never fun. The hardest part is locating which bolts have to be removed but the wiring can also be a mess. If there is water leaking in the floor of your car, do NOT assume that the heater core is leaking. My Grand Prix did this and I found out it was a clogged drain pipe. The black plastic box has a drain to allow condensation from the a/c to drain on the ground which is why you have water puddles after you park your car and have been running the a/c or defroster. If this drain pipe becomes clogged, the condensation will not drain and the water will spill into your floor inside the car. The first thing I would do is to look for a small rubber hose coming from the firewall (in the engine compartment) around the middle of the car about the height of the bottom of the dash. This is the drain hose I referenced earlier. Take compressed air and gently blow back through it to make sure it is open and see if the water stops coming into your car. You can also use a small wire or something similar to stick up in there to clear any clogs, but be careful not to poke the heater or a/c core! (email@example.com)
Edit - I am working on replacing my heater core on my 1993 Grand Prix currently. In order to reach it, you must remove the ashtray. It helps to disconnect the wires, there are only 2, so it isn't dangling in your way while you lay upside down on your front seat under your dash. There are only two or four bolts to remove the ashtray - its quite simple.
It also helps to remove the interior fascia that covers the rear of the footwells. Use a med. Phillips head to remove the plastic screws and these will pop right out. You also have to remove the fascia covering the back panel below the ashtray. Feel around for the screws... a 1/4 in drive ratchet w/ a 7mm socket are a HUGE Help. You'll also need a 1/4 extension. I found all of the screws by feel, but the thing still takes some... urging? to get it out the rest of the way. This will expose the heater core box... again feel for the join and follow it around for the screws... there are more than you think... This too takes some pushing in prodding to get out once you find the screws, but once it's off, the heater core is revealed... CAREFUL!!! I'm assuming you're doing this cause your heater core is leaking, (why else would you want to?) so there's going to be a boat load of antifreeze in the heater core box. Personally, I wasn't thinking this and dumped about half on my head. Nasty stuff. Move out of the way.
Before removing the heater core from its bracket go under the hood and detatch the hoses from it... they're about a foot in from the drivers side and about a foot down. If you find a simple way to do this, let me know. I've thus far found it impossible to do with out cutting the hose.
The metal tubes are mounted to the fire wall on the interior by one bolt. Undo this, pop the bracets holding the body in, and voila! Your heater core should pull right out.
Good luck in your fixing!