I've had this happen on my 3.1 grand am. Suspected heater core, actually was a rubber hose from motor to firewall that's for the heater core, noticed the 2 hoses back there had never been replaced and were in very bad condition. After replacing them I now understand why noone replaces them till they blow. But the 3100 is a compact motor and the gp's have a much larger engine bay than the grand ams. A 12 pack, a buddy to help drink the twelve pack, and an hour on a Saturday afternoon should do the trick to replace those things. I always start with the easier cheaper simple things and cross my fingers before getting into something as labor intensive as a heater core. If they're screw clamps get some of those super long screw drivers and if they're the type you pinch together to release get a long set of needle nose. You'll probably find that having access underneeth and on top will be neccesary. Remove the hood if you need to. Goodluck!
