540 or 528?
The BIGGEST problem with 5's is the radiator neck cracking on the 540. A pinhole will develop just below the upper neck, and the neck will eventually break off.
528, I'd look at the expansion tank. Just keep the engine off, and fill the expansion tank, you'll probably see it pour out of the side of the tank, or just under the upper lip. If not, unscrew the bleeder screw(s), fill the systm up until water comes out of the bleeders, tighten the bleeders, and run the car with the heater on, and check for your leak. Don't let it get hot though, as there is likely air trapped in the system, and it needs to bleed out.
I'd wager that your defrost quit working because the coolant level was too low to get adequate flow to the heater core....they rarely have any problems at all.
Keep in mind that if the coolant is too low, the temp guage might not read right.