Star the car and let it get warm. The two coolant hoses that come out of the fire wall should both get warm. If one does not, that is the exit flow side, and where the clog needs to be flushed from. I recommend doing a complete flush and fill with radiator cleaner, but before filling it, flush the heater core. Do that by disconnecting the two hoses at the fire wall (should be held on by simple clamps)and then flush it directly FROM the cold hose side (opposite of the direction of the flow) to flush the clog back out the way it came. Use light pressure - like a low/medium flow garden hose, to avoid damaging the heater core. You can do JUST the back flush to unclog the heater core, but the sediment/rust in the system will just reclog it if you don't do a flush/fill of the whole system. Everything you need can be bought at Walmart for under $40 dollars to do a complete flush/fill. Also, be aware that the mid to late 90's Sable and the other 2 cars just like it, this is a COMMON problem and you will need to do this a few times to completely clean it out. Space it out and do it 3 or 4 times over the course of a year.