This morning I turned on my defroster for the first time in about a year (South Florida) and the smell of engine coolant filled the cabin. I'm thinking heater core is bad.... any other thoughts?

 

Heater core. Pull back the carpet and take off that black piece of plastic to check if you can see the anti-freeze dripping.

 

Would a bad heater core also lead to foul, must smelling air conditioning? Cabin filter replaced but that didnt help the a/c smell.

 

Well, you load up your ductwork with damp air then shut off the blower fan and let it cook (you are in Florida where it gets warm), somethings gonna grow eventually.
My smell went away fairly fast but I had the carpet and pads pulled, washed very well and completely dried, washed the floorpan too.(my son poured 2 gallons of premix in before he came home and I found the leak- I suspect most are found quicker than this)

Pain in the butt? Yep. I may change out the core on my S80 at 80K just to avoid it all. I may. Or I'll put it off and kick myself when it busts.

 

Do your windows fog when the heat is on? If so, change that heater core!

 

No, windows do not fog. Only time heat is used is to defrost windows in winter mornings. There is no evidence of the heater core leaking either. Anybody know where the evaporator drain lines are on the car? Maybe i can clean them out some how?