1998 Ford Ranger Heater Problem

I recently had my thermostat replaced in my 1998 Ford Ranger because the inside gauge on the dash was moving up to the top of the gauge into the red zone. Now I can't control the temperature, it blows hot air all the time. It didn't matter where I turned the temp dial on the dash. Just hot air. Fortunately I can turn the fan completely off. I took it back in and they are telling me that it might be the heater box door at the tune of 700. To 1200. To be replaced. 7-8 hour job I'm told it will take to fix the problem. It seems strange to me because I didn't have a problem until I had the thermostat replaced. Could it be something else or is it what they are telling me.