If you are going to replace your heater core yourself like i recently did you should know that it can be done without draining the A/C system or removing the front seats. Before i replaced mine i read up on it everywhere trying to figure out how to do it and everyone said it was an 8 to 10 hour job and most mechanics charge an arm and a leg for it. I replaced mine in about 6.5 hours total and without draining the A/C.
If your not afraid to take a dremel to your air plenum all you have to do is cut the lid just to your right of the heater core otherwise you would have to drain the A/C remove the whole air plenum just to get the lid off. Sorry i dont have more details and pics but im sure if your willing to take on the job you can just wing it like i did. Here is a pic just to help u realize the amount of disassembly though it wasnt completely disassembled when i took the pic.
I hope this saves someone some time and money if you have any question i will try to help the best i can.heater core.jpg