Sometimes the heat in my 1998 Cherokee works, sometimes not. Lately not so much. Someone mentioned that it might be my thermostat. Any truth to this? If so, is this an easy fix? Besides not getting any heat in my cherokee, is there anyway to perform some type of diagnostic to tell if the heater core has failed? I'm told that this is a pricey repair at the dealer and not to try doing this on my own. Can someone help? Thanks.