Mitsubishi Repair: mitsubishi gallant 2001, mitsubishi gallant, heater core

Question
how do i remove the heater core

Answer
Alvaro,
If you really want to remove your heater core yourself, you should pick yourself up a service manual for your car.  It will show detailed instruction on the removal process.  Both Haynes and Chiltons make service manuals for your car.

But removal will involve, first, removing most of your passenger side dashboard and glove box.  Also, you would have to remove your A/C evaporator; which would involve evacuating your A/C refrigerant.  Removing refrigerant should be done at a professional A/C service shop; especially when you have to refill the refrigerant.  

Once the refrigerant has been removed, you will need to drain your coolant before pulling out the heater core.  So after all of those items have been removed, then you'll be able to pull out your heater core.  It's not an easy job and I wouldn't recommend doing it yourself unless you have experience with auto repair.  It's a labor intensive process.
Good luck!