Those 3.0 Taurusses have a problem with rusty deposits in the coolant system. I have to rinse my system every year. I would suggest Jackie to have her heater core flushed again I both directions. I usually use a flush kit and put the garden hose on the hose coming from the water pump, with the hose going back into the engine block (thermostat side) disconnected. Then connect the garden hose on the hose (thermostat side). Worth another try before spending $600.00!!!
A flush kit only costs about $10.00. And it only takes 45 minutes to flush and refill the system.