Heater core problem?

Need help in diagnosing a problem with a 87 Buick Park Avenue.

Car overheats and coats the inside windshield with what looks like antifreeze. This happens with or without the a/c or heater on. Sounds to me like the heater core, but what about the thermostat? could this also cause these symptoms?

Also need help in locating both the thermostat and heater core in this car as well. Any info you can give me would be greatly appreciated. Thanks in advance!

The thermostat is located where the radiator hose goes into the engine. But it sounds to me (I'm NOT a mechanic) like the heater core is leaking. It's located behind the firewall, or from the inside of the car, behind the glove box. On my 92, it's not easy to get to.

Sounds like your coolant system has been leaking for some time and the level may be down causing the overheat situation. Check your rad level by removing the rad cap when cold. Even if there's coolant in the overflow bottle the overflow tube may be blocked and it's not drawing from the bottle (my experience).If you take the bottom silencer panel off the dash you can see the bottom of the heater box behind the glove box. Look for coolant seepeing out of the joint in the box. You may have to loosen it since it is sealed. You can inspect the core by opening the heater box a few inches. Replacement of the heater core is a difficult job. Wreckers will sell you a used one but you might as well buy new since they'l probably damage the used one getting it out. They're cheap too so buy new. ($65).