Hi Carol,what makes you think your heater core is leaking,is your antifreeze container low,if not then it's not leaking,could cost 3 to 5 hundred bucks,I just changed mine in my truck,it was leaking on driver side and my windows were getting fogged up.I had to lift the dash up on mine to do,i did it 10 years ago,so this time was easy,here's a link on replacing your heater core,Does your heater get the inside warm,does it have a musky smell?if it is leaking,then your antifreeze container will be low,has it been raining alot,you said in your other post that water is on the passenger side,rain water can come thru the intake vent.
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