Insulated box with heater, thermostat failure! 125F

[nerdybrewer]

My fermentation box has been working great until the past couple days.
I put a small electric heater inside and the thermostat on the heater has been controlling the temperature inside.
Until it stuck.
I checked this morning and it was 125F inside and the barrel of barley mash was at about 115F while the rum wash was at about 105F.
After they cool off I'll check the SG on both, they were nearly done, hope they didn't get too messed up by the high temp.
Each is about 18 gallons, that's a lot of stilling I need to get started on soon.

I've wired up a line voltage house thermostat so I can plug a heater into it, I will no longer rely on the heater to control itself.