Pick up a Haynes or Chiltons repair manual at your local auto parts store specific to your vehicle.
Every question you might have can be answerd in that book
How do i change the heater core in a 1989 chevy corsica?
Somebody else asked about this a few days ago. It's all in Chiltons Auto Repair manual. I want you to see for yourself, so you can judge whether or not you want to do it or take itto a shop.
