You may not need to replace the heater core; there are two O-rings when the coolant hoses attach to the core and these can be your culprit. I had a similar issue with my 01 with coolant leaking on driver side floor. I replaced the O-rings several years ago and have had no issues since then. The 01 h...

This is by far the best post regarding this that I have seen. Pictures are great and will be a good reference in the future when I decide to do this to my 01 (again as I have done it before with tube). I am bookmarking this. Thanks!