My car is over heating and it is blocked at the heater core.

We know this because we have done everything else to the heating system. All that is left is the heater core so we need to change it out and I need to know how to do this?? I now that it is a really big and hard job but it has to be done,so with a little help from you guys maybe we can get the job done.We have tried running CLR though it and for awhile that was working but I think that there is just to much build-up in the heater-core now and it is 100 percent blocked. We also tried flushing it out a few times and that did not work either.So know we come to you guys and hope that we can get this problem fixed.