1993 Ford Explorer Heater core? Belt? Thermostat?

For a couple of days last week I thought I might need a new thermostat, had all the signs, radiator full, over flow full, temp would go up about halfway, after turning on heater it would go right back down. Then after a couple of days it went back to normal no more signs so I figured clogged or was stuck? Last 4-5 days no problems. This morning turned defrost on and my windows got foggier and I could smell radiator. Im thinking its now heater core? Im single mom so need to find problem without spending money on something thats not the problem. What do you think it is?